Display device

ABSTRACT

A display device including an oxide semiconductor, a protective circuit and the like having appropriate structures and a small occupied area is necessary. The protective circuit is formed using a non-linear element which includes a gate insulating film covering a gate electrode; a first oxide semiconductor layer which is over the gate insulating layer and overlaps with the gate electrode; and a first wiring layer and a second wiring layer each of which is formed by stacking a conductive layer and a second oxide semiconductor layer and whose end portions are over the first oxide semiconductor layer and overlap with the gate electrode. The gate electrode of the non-linear element is connected to a scan line or a signal line, the first wiring layer or the second wiring layer of the non-linear element is directly connected to the gate electrode layer so as to apply potential of the gate electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device including an oxidesemiconductor.

2. Description of the Related Art

A thin film transistor formed over a flat plate such as a glasssubstrate is manufactured using amorphous silicon or polycrystallinesilicon, as typically seen in a liquid crystal display device. Thin filmtransistors manufactured using amorphous silicon have low field effectmobility, but can be formed over a glass substrate with a larger area.On the other hand, thin film transistor manufactured usingpolycrystalline silicon have high field effect mobility, but acrystallization step such as laser annealing is necessary and are notalways suitable for a larger glass substrate.

In view of the foregoing, attention has been drawn to a technique formanufacturing a thin film transistor using an oxide semiconductor, andapplying the transistor to an electronic device or an optical device.For example, Patent Document 1 and Patent Document 2 disclose atechnique by which a thin film transistor is manufactured using an oxidesemiconductor containing zinc oxide (ZnO) or containing indium, gallium,and zinc as an oxide semiconductor film and such a transistor is used asa switching element or the like of an image display device.

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

SUMMARY OF THE INVENTION

A thin film transistor in which a channel formation region is formedusing an oxide semiconductor has properties as follows: the operationspeed is higher than that of a thin film transistor including amorphoussilicon and the manufacturing process is simpler than that of a thinfilm transistor including polycrystalline silicon. That is, the use ofan oxide semiconductor makes it possible to manufacture a thin filmtransistor with high field effect mobility even at low processtemperatures ranging from room temperature to 300° C. or lower.

In order to take advantage of the properties of and to ensure thereliability of a display device including an oxide semiconductor, whichis superior in operating characteristics and can be manufactured at lowtemperatures, it is necessary that a protective circuit and the likehaving appropriate structures are formed in the display device.Moreover, it is necessary to reduce an area occupied by the protectivecircuit for achieving reduction in size of the display device.

According to one mode of the present invention, an object is to providea structure which is suitable for a protective circuit.

According to one mode of the present invention, another object of thepresent invention is to enhance the function of a protective circuit,stabilize the operation, and achieve decrease in area occupied by theprotective circuit in a display device intended for a variety ofpurposes manufactured by stacking, in addition to an oxidesemiconductor, an insulating film and a conductive film.

An embodiment of the present invention is a display device in which aprotective circuit is formed using a non-linear element including anoxide semiconductor. This non-linear element includes a combination ofoxide semiconductors with different oxygen contents. Further, at leastone of a first wiring layer and a second wiring layer included in thenon-linear element is directly connected to a gate electrode or aconductive film formed in the same step as the gate electrode layer.

An illustrative embodiment of the present invention is a display devicewhich includes scan lines and signal lines provided over a substratehaving an insulating surface so as to intersect with each other, a pixelportion in which pixel electrodes are arranged in matrix, and anon-linear element formed using an oxide semiconductor in a regionoutside the pixel portion. The pixel portion includes a thin filmtransistor in which a channel formation region is formed in a firstoxide semiconductor layer. The thin film transistor in the pixel portionincludes a gate electrode which is connected to the scan line, a firstwiring layer which is connected to the signal line and which is incontact with the first oxide semiconductor layer, and a second wiringlayer which is connected to the pixel electrode and which is in contactwith the first oxide semiconductor layer. Moreover, the non-linearelement is provided between the pixel portion and a signal inputterminal disposed at the periphery of the substrate. The non-linearelement includes a gate electrode; a gate insulating layer covering thegate electrode; a first oxide semiconductor layer which is over the gateinsulating layer and overlaps with the gate electrode; and a firstwiring layer and a second wiring layer each of which is formed bystacking a conductive layer and a second oxide semiconductor layer andwhose end portions are over the first oxide semiconductor layer andoverlap with the gate electrode. Further, the gate electrode of thenon-linear element is connected to the scan line or the signal line, andthe first wiring layer or the second wiring layer of the non-linearelement is directly connected to the gate electrode so as to applypotential of the gate electrode.

Another illustrative embodiment of the present invention is a displaydevice which includes scan lines and signal lines provided over asubstrate having an insulating surface so as to intersect with eachother, a pixel portion in which pixel electrodes are arranged in matrix,and a protective circuit in a region outside the pixel portion. Thepixel portion includes a thin film transistor in which a channelformation region is formed in a first oxide semiconductor layer. Thethin film transistor in the pixel portion includes a gate electrodewhich is connected to the scan line, a first wiring layer which isconnected to the signal line and which is in contact with the firstoxide semiconductor layer, and a second wiring layer which is connectedto the pixel electrode and which is in contact with the first oxidesemiconductor layer. In the region outside the pixel portion, aprotective circuit connecting the scan line and a common wiring to eachother and a protective circuit connecting the signal line and a commonwiring to each other are provided. Each protective circuit includes anon-linear element including a gate electrode; a gate insulating layercovering the gate electrode; a first oxide semiconductor layer which isover the gate insulating layer and overlaps with the gate electrode; anda first wiring layer and a second wiring layer each of which is formedby stacking a conductive layer and a second oxide semiconductor layerand whose end portions are over the first oxide semiconductor layer andoverlap with the gate electrode. Further, the gate electrode of thenon-linear element included in the protective circuit is directlyconnected to the first wiring layer or the second wiring layer.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify theinvention.

According to an embodiment of the present invention, a protectivecircuit is formed using a non-linear element including an oxidesemiconductor, whereby a display device having a structure suitable fora protective circuit can be obtained. In a connection structure betweenthe first oxide semiconductor layer and the wiring layer of thenon-linear element, the provision of the region which has a contact withthe second oxide semiconductor layer, which has higher electricalconductivity than the first oxide semiconductor layer, allows stableoperation. Accordingly, the function of the protective circuit isenhanced and the operation can be made stable.

Alternatively, there is a method in which the first or second wiringlayer and the gate electrode or the wiring formed with the same layer asthe gate electrode are connected using another wiring layer by providinga contact hole reaching the first or second wiring layer and a contacthole reaching the gate electrode or the wiring formed with the samelayer as the gate electrode. However, two interfaces and two contactholes are formed for one connection in this method.

In the non-linear element included in the protective circuit accordingto an embodiment of the present invention, the conductive layer of thefirst or second wiring layer is directly connected to the gate electrodeor the wiring formed with the same layer as the gate electrode; thus,only one interface and one contact hole are formed for one connection.The connection method enables contact resistance to be reduced ascompared with the connection method using another wiring layer becausethe number of interfaces formed for one connection is only one.Accordingly, the protective circuit including the non-linear elementoperates stably. In addition, the connection method of the presentinvention enables an area occupied by the connection portion to bereduced as compared with the connection method using another wiringbecause only one contact hole is needed for connection. Accordingly, anarea occupied by the protective circuit can be reduced and reduction insize of the display device can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a positional relationship among signal inputterminals, scan lines, signal lines, protective circuits includingnon-linear elements, and a pixel portion in a display device.

FIG. 2 illustrates an example of a protective circuit.

FIG. 3 illustrates an example of a protective circuit.

FIGS. 4A and 4B are a plan view and a circuit diagram illustrating anexample of a protective circuit.

FIG. 5 is a cross-sectional view illustrating an example of a protectivecircuit.

FIGS. 6A to 6C are cross-sectional views illustrating a process formanufacturing a protective circuit.

FIGS. 7A to 7C are cross-sectional views illustrating a process formanufacturing a protective circuit.

FIGS. 8A and 8B are a plan view and a circuit diagram illustrating anexample of a protective circuit.

FIGS. 9A and 9B are a plan view and a circuit diagram illustrating anexample of a protective circuit.

FIG. 10 is a cross-sectional view of an electronic paper.

FIGS. 11A and 11B are each a block diagram of a semiconductor device.

FIG. 12 illustrates a structure of a signal line driver circuit.

FIG. 13 is a timing chart of operation of a signal line driver circuit.

FIG. 14 is a timing chart of operation of a signal line driver circuit.

FIG. 15 is a diagram illustrating a structure of a shift register.

FIG. 16 illustrates a connection structure of a flip-flop of FIG. 14.

FIGS. 17A1 and 17A2 are top views and FIG. 17B is a cross-sectionalview, each illustrating a semiconductor device of an embodiment.

FIG. 18 is a cross-sectional view illustrating a semiconductor device ofan embodiment.

FIG. 19 illustrates an equivalent circuit of a pixel in a semiconductordevice of an embodiment.

FIGS. 20A to 20C each illustrate a semiconductor device of anembodiment.

FIG. 21A is a top view and FIG. 21B is a cross-sectional viewillustrating a semiconductor device of an embodiment.

FIGS. 22A and 22B illustrate examples of applications of an electronicpaper.

FIG. 23 is an external view illustrating an example of an electronicbook reader.

FIG. 24A is an external view of an example of a television device andFIG. 24B is an external view of an example of a digital photo frame.

FIGS. 25A and 25B are external views each illustrating examples of gamemachines.

FIG. 26 is an external view illustrating an example of a cellular phone.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention are described belowwith reference to the drawings. The present invention is not limited tothe description below and it is easily understood by those skilled inthe art that the mode and details can be changed variously withoutdeparting from the scope and spirit of the present invention. Therefore,the present invention should not be interpreted as being limited to whatis described in the embodiments described below. Note that a referencenumeral denoting the same portion in different figures is used in commonin the structures of the present invention explained below.

Embodiment 1

In this embodiment, an example of a display device including a pixelportion and a protective circuit which includes a non-linear elementprovided near the pixel portion is described with reference to drawings.

FIG. 1 illustrates a positional relationship among signal inputterminals, scan lines, signal lines, protective circuits includingnon-linear elements, and a pixel portion in a display device. Over asubstrate 10 having an insulating surface, scan lines 13 and signallines 14 intersect with each other to form a pixel portion 17.

The pixel portion 17 includes a plurality of pixels 18 arranged inmatrix. The pixel 18 includes a pixel transistor 19 connected to thescan line 13 and the signal line 14, a storage capacitor portion 20, anda pixel electrode 21.

In the pixel structure illustrated here, one electrode of the storagecapacitor portion 20 is connected to the pixel transistor 19 and theother electrode is connected to a capacitor line 22. Moreover, the pixelelectrode 21 forms one electrode which drives a display element (such asa liquid crystal element, a light-emitting element, or a contrast medium(electronic ink)). The other electrode of such a display element isconnected to a common terminal 23.

The protective circuit is provided between the pixel portion 17, and ascan line input terminal 11 and a signal line input terminal 12. In thisembodiment, a plurality of protective circuits are provided. Therefore,even though surge voltage due to static electricity and the like isapplied to the scan line 13, the signal line 14, and a capacitor busline 27, the pixel transistor 19 and the like are not broken.Accordingly, the protective circuit has a structure for releasing chargeto a common wiring 29 or a common wiring 28 when surge voltage isapplied to the protective circuit.

In this embodiment, a protective circuit 24, a protective circuit 25,and a protective circuit 26 are provided in the display device. Needlessto say, the structures of the protective circuits are not limited tothose above.

FIG. 2 illustrates an example of the protective circuit. This protectivecircuit includes a non-linear element 30 and a non-linear element 31which are arranged in parallel between the scan line 13 and the commonwiring 29. Each of the non-linear element 30 and the non-linear element31 is a two-terminal element such as a diode or a three-terminal elementsuch as a transistor. For example, the non-linear element can be formedthrough the same steps as the pixel transistor of the pixel portion. Forexample, characteristics similar to those of a diode can be obtained byconnecting a gate terminal to a drain terminal of the non-linearelement.

A first terminal (gate) and a third terminal (drain) of the non-linearelement 30 are connected to the scan line 13, and a second terminal(source) thereof is connected to the common wiring 29. A first terminal(gate) and a third terminal (drain) of the non-linear element 31 areconnected to the common wiring 29, and a second terminal (source)thereof is connected to the scan line 13. That is, the protectivecircuit illustrated in FIG. 2 includes two transistors whose rectifyingdirections are opposite to each other and which connect the scan line 13and the common wiring 29 to each other. In other words, the protectivecircuit has a structure in which between the scan line 13 and the commonwiring 29, a transistor whose rectifying direction is from the scan line13 to the common wiring 29 and a transistor whose rectifying directionis from the common wiring 29 to the scan line 13 are connected.

In the protective circuit illustrated in FIG. 2, when the scan line 13is charged positively or negatively with respect to the common wiring 29due to static electricity or the like, current flows in a direction thatcancels the charge. For example, if the scan line 13 is positivelycharged, current flows in a direction in which the positive charge isreleased to the common wiring 29. Owing to this operation, electrostaticbreakdown or a shift in the threshold voltage of the pixel transistor 19connected to the charged scan line 13 can be prevented. Moreover, it ispossible to prevent dielectric breakdown of an insulating layer betweenthe charged scan line 13 and another wiring that intersects with thecharged scan line 13 with the insulating layer interposed therebetween.

Note that in FIG. 2, a pair of the non-linear elements the rectifyingdirections of which are opposite to each other are used: the non-linearelement 30 whose first terminal (gate) is connected to the scan line 13and the non-linear element 31 whose first terminal (gate) is connectedto the common wiring 29. The common wiring 29 and the scan line 13 areconnected via the second terminal (source) and the third terminal(drain) of each non-linear element; that is, the non-linear element 30and the non-linear element 31 are in parallel. As another structure, anon-linear element may be further added in parallel connection, so thatthe operation stability of the protective circuit can be enhanced. Forexample, FIG. 3 illustrates a protective circuit provided between thescan line 13 and the common wiring 29 which includes a non-linearelement 30 a and a non-linear element 30 b, and a non-linear element 31a and a non-linear element 31 b. This protective circuit includes fournon-linear elements in total: two non-linear elements (30 b and 31 b)each having a first terminal (gate) which is connected to the commonwiring 29 and two non-linear elements (30 a and 31 a) each having afirst terminal (gate) which is connected to the scan line 13. That is tosay, two pairs of non-linear elements are connected between the commonwiring 29 and the scan line 13, each pair including two non-linearelements provided so that their rectifying directions are opposite toeach other. In other words, between the scan line 13 and the commonwiring 29, there are two transistors, rectifying direction of which arefrom the scan line 13 to the common wiring 29 and two transistors,rectifying direction of which are from the common wiring 29 to the scanline 13. When the common wiring 29 and the scan line 13 are connected toeach other with the four non-linear elements in this manner, it ispossible to prevent charge from directly flowing through the scan line13, even if surge voltage is applied to the scan line 13 and moreovereven if the common wiring 29 is charged by static electricity or thelike. Note that FIG. 9A illustrates an example in which four non-linearelements are provided over a substrate and FIG. 9B is an equivalentcircuit diagram thereof. The equivalent circuit diagram of FIG. 9Bcorresponds to that of FIG. 3, and the non-linear elements illustratedin FIG. 9B correspond to the non-linear elements illustrated in FIG. 3.Specifically, the non-linear element 740 a corresponds to the non-linearelement 30 b; the non-linear element 740 b, the non-linear element 31 b;the non-linear element 740 c, the non-linear element 30 a; and thenon-linear element 740 d, the non-linear element 31 a. Moreover, a scanline 651 and a common wiring 650 in FIGS. 9A and 9B correspond to thescan line 13 and the common wiring 29 in FIG. 3, respectively.Accordingly, the protective circuit illustrated in FIG. 9A in which fournon-linear elements are provided over a substrate is another example ofthe protective circuit illustrated in FIG. 3.

FIG. 8A illustrates an example of a protective circuit which is formedusing an odd number of non-linear elements over a substrate, and FIG. 8Bis an equivalent circuit diagram thereof. In this circuit, a non-linearelement 730 b and a non-linear element 730 a are connected to anon-linear element 730 c to serve as switching elements. By the serialconnection of the non-linear elements in this manner, instantaneous loadapplied to the non-linear elements of the protective circuit can bedeconcentrated.

FIG. 2 illustrates the protective circuit which is provided for the scanline 13;

however, a protective circuit with a similar structure can be providedfor the signal line 14.

FIG. 4A is a plan view illustrating an example of a protective circuitand FIG. 4B is an equivalent circuit diagram thereof. FIG. 5 is across-sectional view taken along line Q1-Q2 of FIG. 4A. A structureexample of the protective circuit is described below with reference toFIGS. 4A and 4B and FIG. 5.

A non-linear element 170 a and a non-linear element 170 b respectivelyinclude a gate electrode 111 and a gate electrode 16, which are formedusing the same layer as the scan line 13. A gate insulating film 102 isformed over the gate electrode 111 and the gate electrode 16. Over thegate insulating film 102, a first oxide semiconductor layer 113 isformed. A first wiring layer 117 a and a second wiring layer 117 b areprovided so as to face each other over the gate electrode 111 with thefirst oxide semiconductor layer 113 interposed between the first andsecond wiring layers 117 a and 117 b and the gate electrode 111. Notethat, main parts of the non-linear element 170 a and the non-linearelement 170 b have the same structure.

In this embodiment of the present invention, the scan line 13 formedusing the same layer as the gate electrode 111 and the third terminal(drain) of the non-linear element 170 a are directly connected through acontact hole 128 provided in the gate insulating film 102. Thus, thenumber of interfaces formed for connection can be reduced to be one, andthe number of contact holes formed for one/the connection can be reducedto be one.

The first oxide semiconductor layer 113 is provided under the firstwiring layer 117 a and the second wiring layer 117 b facing each otherand covers the gate electrode 111 with the gate insulating film 102interposed therebetween. In other words, the first oxide semiconductorlayer 113 is provided so as to overlap with the gate electrode 111 andto be in contact with an upper surface portion of the gate insulatingfilm 102 and lower surface portions of second oxide semiconductor layers114 a and 114 b. Here, the first wiring layer 117 a has a structure inwhich the second oxide semiconductor layer 114 a and a conductive layer115 a are stacked in that order over the first oxide semiconductor layer113. Similarly, the second wiring layer 117 b has a structure in whichthe second oxide semiconductor layer 114 b and a conductive layer 115 bare stacked in that order over the first oxide semiconductor layer 113.

The second oxide semiconductor layer (114 a and 114 b) is providedbetween and in contact with the conductive layer (115 a and 115 b) andthe first oxide semiconductor layer 113. Thus, the oxide semiconductorlayers having different physical properties have contact with eachother, that is, the first oxide semiconductor layer 113 and the secondoxide semiconductor layer (114 a and 114 b) having higher electricalconductivity than the first oxide semiconductor layer 113 are in contactwith each other. Such a contact structure is provided in the non-linearelement 170 a and the non-linear element 170 b, whereby stable operationbecomes possible. In other words, the thermal stability is increased, sothat the stable operation becomes possible. Accordingly, the function ofthe protective circuit is enhanced and then operation can be stabilized.In addition, the amount of junction leakage is reduced and thecharacteristics of the non-linear element 170 a and the non-linearelement 170 b can be improved.

In this specification, a thin film whose composition formula isrepresented as InMO₃(ZnO)_(m) (m>0) is formed as an oxide semiconductorused for the first oxide semiconductor layer, and a non-linear elementand a thin film transistor are formed using the thin film as asemiconductor layer. Note that M denotes one or more metal elementsselected from Ga, Fe, Ni, Mn, and Co. In addition to a case where onlyGa is contained as M, there is a case where Ga and any of the abovemetal elements other than Ga, for example, Ga and Ni or Ga and Fe arecontained as M. Further, in some cases, the above oxide semiconductorcontains a transition metal element such as Fe or Ni or an oxide of thetransition metal as an impurity element in addition to the metal elementwhich is contained as M. In this specification, this thin film is alsoreferred to as an In—Ga—Zn—O based non-single-crystal film.

Table 1 shows a typical measurement example by an Inductively CoupledPlasma Mass Spectrometry (ICP-MS) method. An oxide semiconductor film ofInGa_(0.95)Zn_(0.41)O_(3.33) is obtained under Condition 1 where a flowrate of an argon gas in sputtering is set at 40 sccm, with use of atarget (In: Ga:Zn=1:1:0.5) which contains indium oxide (In₂O₃), galliumoxide (Ga₂O₃), and zinc oxide (ZnO) at a ratio of 1:1:1(═In₂O₃:Ga₂O₃:ZnO). Further, an oxide semiconductor film ofInGa_(0.94)Zn_(0.4)O_(3.31) is obtained by employing Condition 2 whereflow rates of an argon gas and oxygen in sputtering are set at 10 sccmand 5 sccm, respectively.

TABLE 1 Flow ratio Composition (atomic %) Ar/O₂ In Ga Zn O Compositionformula 40/0 17.6 16.7 7.2 58.6 InGa_(0.95)Zn_(0.41)O_(3.33) 10/5 17.716.7 7 58.6 InGa_(0.94)Zn_(0.40)O_(3.31)

Table 2 shows a measurement result of quantification which is performedby a Rutherford Backscattering Spectrometry (RBS) method instead of anICP-MS method.

TABLE 2 Flow ratio Composition (atomic %) Ar/O₂ In Ga Zn O ArComposition formula 40/0 17 15.8 7.5 59.4 0.3InGa_(0.93)Zn_(0.44)O_(3.49) 10/5 16 14.7 7.2 61.7 0.4InGa_(0.92)Zn_(0.45)O_(3.86)

As a result of measuring the sample of Condition 1 by RBS, the oxidesemiconductor film is represented as InGa_(0.93)Zn_(0.44)O_(3.49).Further, as a result of measuring the sample of Condition 2 by RBS, theoxide semiconductor film is represented as InGa_(0.92)Zn_(0.45)O_(3.86).

As a crystalline structure of the In—Ga—Zn—O based non-single-crystalfilm, an amorphous structure is observed by X-ray diffraction (XRD)spectrometry. Note that In—Ga—Zn—O based non-single-crystal film of theexamined sample is subjected to heat treatment at 200° C. to 500° C.,typically 300° C. to 400° C. for 10 minutes to 100 minutes after filmformation by a sputtering method. Further, a thin film transistor havingelectric characteristics such as an on/off ratio of 10⁹ or higher andmobility of 10 cm²N·S or higher at a gate voltage of ±20 V can bemanufactured.

The second oxide semiconductor layer (114 a and 114 b) has higherelectrical conductivity than the first oxide semiconductor layer 113.Thus, the second oxide semiconductor layer (114 a and 114 b) has afunction similar to source and drain regions of a transistor in thenon-linear element 170 a and the non-linear element 170 b described inthis embodiment. The second oxide semiconductor layer (114 a and 114 b)which is to be source and drain regions has n-type conductivity andactivation energy (ΔE) which is from 0.01 eV to 0.1 eV inclusive and canbe also referred to as an n⁺ region. In the case where the second oxidesemiconductor layer is an non-single-crystal oxide semiconductor layercontaining In, Ga, Zn, and O, nanocrystal is included in thenon-single-crystal structure in some cases.

An interlayer insulating film 107 is provided over the first oxidesemiconductor layer 113. The interlayer insulating film 107 is formed ofan oxide such as silicon oxide or aluminum oxide. Further, by stackingsilicon nitride, aluminum nitride, silicon oxynitride, or aluminumoxynitride over silicon oxide or aluminum oxide, the function of theinterlayer insulating film as a protective film can be enhanced.

In any case, the interlayer insulating film 107 being in contact withthe first oxide semiconductor layer 113 is an oxide layer, whereby it ispossible to prevent oxygen from being extracted from the first oxidesemiconductor layer 113 and prevent the first oxide semiconductor layer113 from changing into an oxygen-deficiency type. Moreover, in the caseof having a structure where the first oxide semiconductor layer 113 isnot in direct contact with an insulating layer including nitride, it ispossible to prevent hydrogen in the nitride from diffusing and causingdefects in the first oxide semiconductor layer 113 due to hydroxylgroups or the like.

According to this embodiment, a display device including a protectivecircuit including an oxide semiconductor can be obtained. A region wherethe conductive layer and the first oxide semiconductor layer havecontact with each other with the second oxide semiconductor layerinterposed therebetween which has higher electrical conductivity thanthe first oxide semiconductor layer, whereby stable operation becomespossible. Therefore, the function of the protective circuit can beenhanced and the operation can be stabilized. Further, the thirdterminal (drain) of the non-linear element 170 a and the scan line 13formed with the same layer as the gate electrode 111 are directlyconnected through the contact hole 128 provided in the gate insulatingfilm 102, whereby formation of one connection needs only one interfaceand only one contact hole. As a result, in addition to enhancingfunction of the protective circuit and achieving stabilization ofoperation, an area occupied by the protective circuit is reduced so thatreduction in size of the display device can be achieved. In particular,as the number of non-linear elements included in the protective circuitis increased to three or four, effect of reducing the numbers ofinterfaces and contact holes is increased.

Note that although FIGS. 4A and 4B and FIG. 5 illustrate the example ofa protective circuit provided at the scan line 13, a similar protectivecircuit can be provided for a signal line, a capacitor bus line, or thelike.

This embodiment can be implemented in combination with a structuredescribed in another embodiment as appropriate.

Embodiment 2

In this embodiment, an example of a process for manufacturing theprotective circuit illustrated in FIG. 4A described in Embodiment 1 isdescribed with reference to FIGS. 6A to 6C and FIGS. 7A to 7C. FIGS. 6Ato 6C and FIGS. 7A to 7C are cross-sectional views taken along lineQ1-Q2 of FIG. 4A.

In FIG. 6A, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, aluminosilicate glass, or the like availablein the market can be used as a substrate 100 having a light-transmittingproperty. For example, a glass substrate which includes more bariumoxide (BaO) than boric acid (B₂O₃) in composition ratio and whose strainpoint is 730° C. or higher is preferable. This is because such a glasssubstrate is not strained even in the case where the oxide semiconductorlayer is thermally processed at high temperatures of about 700° C.

Next, a conductive film which is to be a gate wiring including the gateelectrode 111 and the scan line 13, a capacitor wiring, and a terminalof a terminal portion is formed entirely over the substrate 100. Theconductive film is desirably formed from a low-resistance conductivematerial such as aluminum (Al) or copper (Cu); however, since Al itselfhas disadvantages such as low heat resistance and a tendency to becorroded, it is used in combination with a conductive material havingheat resistance. As a conductive material having heat resistance, anelement selected from titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), or scandium (Sc), or analloy including any of the elements, an alloy film including acombination of such elements, or a nitride film including any of theelements can be used.

The conductive film to be a wiring layer including the gate electrode111 is formed to a thickness of from 50 nm to 300 nm inclusive. When theconductive film to be the wiring layer including the gate electrode 111has a thickness of 300 nm or less, disconnection of a semiconductor filmor a wiring which is formed later can be prevented. Further, when theconductive film to be the wiring layer including the gate electrode 111has a thickness of 150 nm or more, resistance of the gate electrode canbe reduced, and increase in size becomes possible.

Here, a film containing aluminum as its main component and a titaniumfilm are stacked as the conductive film over an entire surface of thesubstrate 100 by a sputtering method.

Next, with use of a resist mask formed using a first photomask in thisembodiment, an unnecessary portion of the conductive film formed overthe substrate is removed by etching, so that a wiring and an electrode(a gate wiring including the gate electrode 111, a capacitor wiring, anda terminal) are formed. At this point, etching is performed so that atleast an end portion of the gate electrode 111 can be tapered. FIG. 6Aillustrates a cross-sectional view at this step.

Then, the gate insulating film 102 is formed. As an insulating filmwhich can be used as the gate insulating film 102, a silicon oxide film,a silicon nitride film, a silicon oxynitride film, a silicon nitrideoxide film, an aluminum oxide film, a magnesium oxide film, an aluminumnitride film, an yttrium oxide film, a hafnium oxide film, or a tantalumoxide film can be given as an example.

Here, a silicon oxynitride film means a film that contains more oxygenthan nitrogen and includes oxygen, nitrogen, silicon, and hydrogen atconcentrations ranging from 55 at. % to 65 at. %, 1 at. % to 20 at. %,25 at. % to 35 at. %, and 0.1 at. % to 10 at. %, respectively. Further,a silicon nitride oxide film means a film that contains more nitrogenthan oxygen and includes oxygen, nitrogen, silicon, and hydrogen atconcentrations ranging from 15 at. % to 30 at. %, 20 at. % to 35 at. %,25 at. % to 35 at. %, and 15 at. % to 25 at. %, respectively.

The gate insulating film may have a single-layer structure or a layeredstructure in which two or three insulating films are stacked. Forexample, when the gate insulating film in contact with the substrate isformed using a silicon nitride film or a silicon nitride oxide film,adhesion between the substrate and the gate insulating film isincreased, and in the case of using a glass substrate as the substrate,impurities from the substrate can be prevented from diffusing into theoxide semiconductor layer and further, oxidation of the wiring layerincluding the gate electrode 111 can be prevented. That is, film peelingcan be prevented, and electric characteristics of the resulting thinfilm transistor can be improved.

The thickness of the gate insulating film 102 is 50 nm to 250 nm. Thegate insulating film with a thickness of 50 nm or more can coverprojections and depressions of the wiring layer including the gateelectrode 111, which is preferable. Here, a 100-nm-thick silicon oxidefilm is formed as the gate insulating film 102 by a plasma CVD method ora sputtering method.

Next, the gate insulating film 102 is etched with use of a resist maskformed using a second photomask in this embodiment, so that the contacthole 128 reaching the scan line 13 is formed.

Then, plasma treatment is performed on the gate insulating film 102prior to formation of a first oxide semiconductor film.

Here, reverse sputtering where plasma is generated after introduction ofan oxygen gas and an argon gas is performed, so that the exposed gateinsulating layer is subjected to treatment using oxygen radicals oroxygen. Thus, dust adhering to the surface is removed.

The plasma treatment of the gate insulating film 102, and formation ofthe first oxide semiconductor film and the second oxide semiconductorfilm by a sputtering method can be performed successively withoutexposure to air. Successive film formation can be performed by changingthe gas introduced to the chamber or the target set in the chamber asappropriate. The successive film formation without exposure to air canprevent impurity mixture. In the case of performing successive filmformation without exposure to air, a manufacturing apparatus of amultichamber type is preferably used.

In particular, it is preferable to successively perform plasma treatmentof the gate insulating film 102 in contact with the first oxidesemiconductor film and formation of the first oxide semiconductor film.By the successive film formation like this, an interface between stackedlayers can be formed without being contaminated by an atmosphericconstituent such as moisture or contaminant elements or dust existing inthe atmosphere. Thus, variations in characteristics of the non-linearelements and thin film transistors can be reduced.

Note that the term “successive film formation” in this specificationmeans that during a series of steps from a first treatment step bysputtering to a second treatment step by sputtering, an atmosphere inwhich a substrate to be processed is disposed is not contaminated by acontaminant atmosphere such as air, and is constantly controlled to bevacuum or an inert gas atmosphere (a nitrogen atmosphere or a rare gasatmosphere). By the successive film formation, film formation can beconducted while moisture or the like is kept from attaching to thecleaned substrate to be processed. Note that successive film formationincludes plasma treatment such as reverse sputtering.

Next, the first oxide semiconductor film is formed in such a manner thatthe gate insulating film 102 which has been subjected to plasmatreatment is not exposed to air. Formation of the first oxidesemiconductor film in such a manner can avoid the trouble that dust ormoisture adheres to the interface between the gate insulating film 102and the first oxide semiconductor film. The first oxide semiconductorfilm may be formed in the chamber where the reverse sputtering isperformed previously, or may be formed in a different chamber from thechamber where the reverse sputtering is performed previously as long asthe film formation can be performed without exposure to air.

Here, the first oxide semiconductor film is formed in an argon or oxygenatmosphere under the condition where the oxide semiconductor targetincluding In, Ga, and Zn (composition ratio is In₂O₃:Ga₂O₃:ZnO=1:1:1)with a diameter of 8 inches is used, the distance between the substrateand the target is set at 170 mm, the pressure is set at 0.4 Pa, and thedirect current (DC) power supply is set at 0.5 kW. Note that a pulsedirect current (DC) power supply is preferable because dust can bereduced and the film thickness can be uniform. The thickness of thefirst oxide semiconductor film is set to 5 nm to 200 nm. The thicknessof the first oxide semiconductor film in this embodiment is 100 nm.

The first oxide semiconductor film is formed under the differentcondition from the second oxide semiconductor film, the first oxidesemiconductor layer has different composition from the second oxidesemiconductor layer; for example, the first oxide semiconductor layerincludes more oxygen than the second oxide semiconductor layer. Forexample, the ratio of the oxygen gas flow rate to the argon gas flowrate in the deposition condition of the first oxide semiconductor filmis set higher than that of the second oxide semiconductor film.Specifically, the second oxide semiconductor film is formed in a raregas (such as argon or helium) atmosphere (or a gas including oxygen at10% or less and argon at 90% or more), while the first oxidesemiconductor film is formed in an oxygen atmosphere (or an atmospherein which a flow rate of an oxygen gas is equal to or more than a flowrate of an argon gas).

When the first oxide semiconductor film contains a large amount ofoxygen, the first oxide semiconductor film can have lower electricalconductivity than the second oxide semiconductor layer. Moreover, whenthe first oxide semiconductor film contains a large amount of oxygen,the amount of off current can be reduced; therefore, a thin filmtransistor with a high on/off ratio can be provided.

Next, the second oxide semiconductor film is formed over the first oxidesemiconductor film by a sputtering method. Here, sputtering depositionis performed under the condition where a 8-inch-diameter target of anoxide semiconductor including In, Ga, and Zn (the ratio ofIn₂O₃:Ga₂O₃:ZnO is 1:1:1) is used, the distance between the target andthe substrate is 170 mm, the pressure in a deposition chamber is set at0.4 Pa, the DC electric power is set at 0.5 kW, the depositiontemperature is set to room temperature, and the argon gas flow rate isset at 40 sccm. Thus, a semiconductor film including In, Ga, Zn, andoxygen as components is formed as the second oxide semiconductor film.Although the target where the composition ratio is In₂O₃:Ga₂O₃:ZnO=1:1:1is used intentionally, an oxide semiconductor film including a crystalgrain which has a size of 1 nm to 10 nm just after the film formation isoften obtained. It can be said that the presence or absence of crystalgrains and the density of crystal grains can be controlled and thediameter of the crystal grain can be adjusted within 1 nm to 10 nm, allby adjusting as appropriate, the deposition condition of reactivesputtering, the target composition ratio, the deposition pressure (0.1Pa to 2.0 Pa), the electric power (250 W to 3000 W: 8 inchesφ), thetemperature (room temperature to 100° C.), and the like. The thicknessof the second oxide semiconductor film is set to 5 nm to 20 nm. Needlessto say, in the case where the film includes crystal grains, the size ofthe crystal grain does not exceed the film thickness. In thisembodiment, the second oxide semiconductor film has a thickness of 5 nm.

Then, a third photolithography process is performed. A resist mask isformed, and the first oxide semiconductor film and the second oxidesemiconductor film are etched. Here, wet etching is performed usingITO07N (product of Kanto Chemical Co., Inc.) to remove an unnecessaryportion; thus, the first oxide semiconductor layer 113 and the secondoxide semiconductor layer 114 are formed. Note that the etching here maybe dry etching, without being limited to wet etching. FIG. 6Billustrates a cross-sectional view at this step.

Next, a conductive film 105 is formed from a metal material over thesecond oxide semiconductor layer 114 and the gate insulating film 102 bya sputtering method or a vacuum evaporation method. As the material ofthe conductive film 105, there are an element selected from Al, Cr, Ta,Ti, Mo, or W, an alloy including any of the elements, an alloy filmincluding a combination of such elements, and the like.

When thermal treatment is conducted at 200° C. to 600° C., theconductive film preferably has heat resistant property so as to endurethis thermal treatment. Since aluminum itself has disadvantages such aslow heat resistance and a tendency to be corroded, it is used incombination with a conductive material having heat resistance. As aconductive material having heat resistance, which is to be used incombination with Al, an element selected from titanium (Ti), tantalum(Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), orscandium (Sc), or an alloy including any of the elements, an alloy filmincluding a combination of such elements, or a nitride film includingany of the elements can be used.

Here, the conductive film 105 has a three-layer structure in which a Tifilm is formed, an aluminum film including Nd, an Al—Nd film, is stackedover the Ti film, and another Ti film is stacked thereover.Alternatively, the conductive film 105 may have a two-layer structure inwhich a titanium film is stacked over an aluminum film. Furtheralternatively, the conductive film 105 may have a single-layer structureof an aluminum film including silicon or a titanium film. FIG. 6Cillustrates a cross-sectional view at this step.

Note that since the contact hole 128 is formed in the gate insulatingfilm 102, the conductive film which is to be source and drain electrodesis connected to the scan line 13 through the contact hole 128 at thesame time as the formation.

Next, a fourth photolithography process is performed. A resist mask 131is formed, and an unnecessary portion of the conductive film 105 isremoved by etching. Thus, conductive layers 115 a and 115 b are formed(see FIG. 7A). At this time, dry etching or wet etching can be used asthe etching. Here, dry etching is employed using a mixed gas of SiCl₄,Cl₂, and BCl₃ to etch the conductive film in which the Al—Nd film andthe Ti films are stacked, so that the conductive layers 115 a and 115 bare formed.

Next, the second oxide semiconductor layer is etched using the resistmask 131, which is used for etching the conductive film 105. Here, wetetching is performed using ITO07N (product of Kanto Chemical Co., Inc.)to remove an unnecessary portion; thus, second oxide semiconductorlayers 114 a and 114 b are formed. Note that the etching at this time isnot limited to wet etching and may be dry etching. In addition, althoughdepending on the etching conditions, a part of an exposed region of thefirst oxide semiconductor layer 113 is also etched in this etching stepof the second oxide semiconductor film. Thus, a channel formation regionof the first oxide semiconductor layer 113 between the second oxidesemiconductor layers 114 a and 114 b is a region with a small thicknessas illustrated in FIG. 7A.

Then, the resist mask 131 is removed. Further, a surface of the exposedregion of the first oxide semiconductor layer 113 may be subjected toplasma treatment. By the plasma treatment, damage of the first oxidesemiconductor layer 113 due to the etching can be repaired. The plasmatreatment is preferably performed in an atmosphere of O₂ or N₂O, orpreferably an atmosphere containing oxygen. The atmosphere containingoxygen refers to, for example, an atmosphere of N₂, He, or Ar to whichoxygen is added. Alternatively, Cl₂ or CF₄ may be added to the aboveatmosphere. Note that the plasma treatment is preferably performedwithout bias. FIG. 7B illustrates a cross-sectional view at this step.

Next, thermal treatment at 200° C. to 600° C., typically 300° C. to 500°C., is preferably performed. In this case, thermal treatment isperformed in a furnace at 350° C. for an hour in a nitrogen atmosphereor an air atmosphere. This thermal treatment allows atoms of theIn—Ga—Zn—O based non-single-crystal film to be rearranged. Since thedistortion that interrupts carrier movement is released by this thermaltreatment, the thermal treatment at this time (includingphoto-annealing) is important. There is no particular limitation on whento perform the thermal treatment as long as it is performed after theformation of the oxide semiconductor film; for example, it may beperformed after the formation of the pixel electrode. Through thesesteps, the non-linear element 170 a in which the first oxidesemiconductor layer 113 is a channel formation region is completed.

Next, an interlayer insulating film 107 covering the non-linear element170 a is formed. The interlayer insulating film 107 can be formed usinga silicon nitride film, a silicon oxide film, a silicon oxynitride film,an aluminum oxide film, a tantalum oxide film, or the like by asputtering method or the like. As described above, through fourphotolithography processes with use of four photomasks, a protectivecircuit including a plurality of non-linear elements (two non-linearelements 170 a and 170 b in this embodiment) can be completed. FIG. 7Cillustrates a cross-sectional view at this step.

A pixel electrode is formed with respect to a thin film transistor whichis formed by the same process as the non-linear element and disposed ina pixel portion of a display device, following the formation of theprotective circuit. First, with use of a fifth resist mask which is notillustrated, a contact hole reaching a drain electrode layer of the thinfilm transistor in the pixel portion, which is also not illustrated, isformed in the interlayer insulating film 107.

Then, the resist mask is removed, and after that, a transparentconductive film is formed. As a material for the transparent conductivefilm, indium oxide (In₂O₃), indium oxide-tin oxide alloy (In₂O₃—SnO₂,abbreviated to ITO), or the like can be given, and the transparentconductive film can be formed by a sputtering method, a vacuumevaporation method, or the like. Etching treatment of such materials isperformed using a chlorinated acid based solution. However, sinceetching of ITO particularly tends to leave residue, an alloy of indiumoxide-zinc oxide (In₂O₃—ZnO) may be used in order to improve etchingprocessability.

Next, a sixth photolithography process is performed. A resist mask isformed, and an unnecessary portion of the transparent conductive film isremoved by etching, so that the pixel electrode is formed. In addition,a capacitor wiring and the pixel electrode form a storage capacitor byusing the gate insulating film 102 and the protective insulating film107 in a capacitor portion as dielectrics. Moreover, the transparentconductive film is left in the terminal portion to form an electrode ora wiring used for connection with an FPC or to form a terminal electrodefor connection which serves as an input terminal of a source wiring.

In such a manner, the pixel electrodes are formed with respect to aplurality of thin film transistors which are formed by the same processas the non-linear element. Thus, the pixel portion including n-channelTFTs and the protective circuit can be manufactured at the same time. Aprovision of a region which has contact with the second oxidesemiconductor layer having higher electrical conductivity than the firstoxide semiconductor layer allows stable operation. Thus, the function ofthe protective circuit is enhanced and operation can be stabilized. Thescan line 13 formed with the same layer as the gate electrode 111 isdirectly connected to the third terminal (drain) of the non-linearelement 170 a through the contact hole 128 provided in the gateinsulating film 102, whereby formation of one connection needs only oneinterface and only one contact hole. As a result, in addition toenhancing the function of the protective circuit and achievingstabilization of operation, an area occupied by the protective circuitis reduced, so that reduction in size of the display device can beachieved. In other words, in accordance with the steps described in thisembodiment, in addition to enhancing the function of the protectivecircuit and achieving stabilization of operation, a board for anactive-matrix display device, on which a protective circuit with smalloccupied area is mounted can be manufactured.

This embodiment can be implemented in combination with a structuredescribed in another embodiment as appropriate.

Embodiment 3

This embodiment describes an example of an electronic paper in which aprotective circuit and a thin film transistor in a pixel portion areprovided over one substrate, as a display device to which an embodimentof the present invention is applied.

FIG. 10 illustrates an active matrix electronic paper as an example of adisplay device to which an embodiment of the present invention isapplied. A thin film transistor 581 used for a semiconductor device canbe manufactured in a manner similar to the non-linear element describedin Embodiment 2. The thin film transistor 581 has excellent electricalcharacteristics and includes a semiconductor layer including asemiconductor oxide containing In, Ga, and Zn.

The electronic paper of FIG. 10 is an example of a display device inwhich a twisting ball display system is employed. The twisting balldisplay system refers to a method in which spherical particles eachcolored in black and white are arranged between a first electrode layerand a second electrode layer which are electrode layers used for adisplay element, and a potential difference is generated between thefirst electrode layer and the second electrode layer to controlorientation of the spherical particles, so that display is performed.

The thin film transistor 581 has a bottom-gate structure in which thesource electrode layer or the drain electrode layer is electricallyconnected to a first electrode layer 587 in an opening formed in aninsulating layer 585. Between the first electrode layer 587 and a secondelectrode layer 588, spherical particles 589 are provided. Eachspherical particle 589 includes a black region 590 a and a white region590 b, and a cavity 594 filled with liquid around the black region 590 aand the white region 590 b. The circumference of the spherical particle589 is filled with filler 595 such as a resin or the like (see FIG. 10).Note that reference numerals 580, 583, 584 and 596 in FIG. 10 denote asubstrate, interlayer insulating layer, protective film and a substrate,respectively.

Further, instead of the twisting ball, an electrophoretic element can beused. A microcapsule with a diameter of about 10 μm to 200 μm which isfilled with transparent liquid, positively-charged white microparticlesand negatively-charged black microparticles is used. In the microcapsulewhich is provided between the first electrode layer and the secondelectrode layer, when an electric field is applied by the firstelectrode layer and the second electrode layer, the white microparticlesand the black microparticles move to opposite sides to each other, sothat white or black can be displayed. A display element using thisprinciple is an electrophoretic display element and is called anelectronic paper in general. The electrophoretic display element hashigher reflectance than a liquid crystal display element, and thus, anassistant light is unnecessary. Moreover, power consumption is low and adisplay portion can be recognized in a dusky place. Furthermore, animage which is displayed once can be retained even when power is notsupplied to the display portion. Accordingly, a displayed image can bestored even when a semiconductor device having a display function (whichis also referred to simply as a display device or a semiconductor deviceprovided with a display device) is distanced from an electric wavesource which serves as a power supply.

The protective circuit mounted in thus manufactured electronic paper hasless contact holes for connection, and therefore, occupies a smallerarea. Additionally, in the protective circuit, a second oxidesemiconductor layer which has higher electrical conductivity than afirst oxide semiconductor layer is provided between the first oxidesemiconductor layer and a wiring layer, which realizes enhancement offunction and stable operation of the protective circuit. Therefore, theelectronic paper of this embodiment which includes such a protectivecircuit is highly reliable.

This embodiment can be implemented in combination with a structuredescribed in another embodiment as appropriate.

Embodiment 4

This embodiment describes, with reference to FIGS. 11A and 11B, FIG. 12,FIG. 13, FIG. 14, FIG. 15, and FIG. 16, a display device which is anexample of a semiconductor device according to an embodiment of thepresent invention. In that display device, at least a protectivecircuit, part of a driver circuit, and a thin film transistor in a pixelportion are formed over one substrate.

The thin film transistor in the pixel portion which is over the samesubstrate as the protective circuit is formed similarly to thenon-linear element described in Embodiment 2. The thin film transistoris formed to be an n-channel TFT; therefore, part of a driver circuitwhich can be formed using n-channel TFTs is formed over the samesubstrate as the thin film transistor in the pixel portion.

FIG. 11A illustrates an example of a block diagram of an active matrixliquid crystal display device which is an example of a semiconductordevice according to an embodiment of the present invention. The displaydevice illustrated in FIG. 11A includes, over a substrate 5300, a pixelportion 5301 including a plurality of pixels each provided with adisplay element; a scan line driver circuit 5302 that selects a pixel;and a signal line driver circuit 5303 that controls a video signal inputto the selected pixel.

The pixel portion 5301 is connected to the signal line driver circuit5303 with a plurality of signal lines S1 to Sm (not shown) extending ina column direction from the signal line driver circuit 5303 andconnected to the scan line driver circuit 5302 with a plurality of scanlines G1 to Gn (not shown) extending in a row direction from the scanline driver circuit 5302. The pixel portion 5301 includes a plurality ofpixels (not shown) arranged in matrix corresponding to the signal linesS1 to Sm and the scan lines G1 to Gn. In addition, each of the pixels isconnected to a signal line Sj (any one of the signal lines S1 to Sm) anda scan line Gi (any one of the scan lines G1 to Gn).

Further, n-channel TFTs can be formed by a method similar to the methodfor forming the non-linear element and together with the non-linearelement described in Embodiment 2. A signal line driver circuitincluding n-channel TFTs is described with reference to FIG. 12.

The signal line driver circuit of FIG. 12 includes a driver IC 5601,switch groups 5602_1 to 5602_M, a first wiring 5611, a second wiring5612, a third wiring 5613, and wirings 5621_1 to 5621_M. Each of theswitch groups 5602_1 to 5602_M includes a first thin film transistor5603 a, a second thin film transistor 5603 b, and a third thin filmtransistor 5603 c.

The driver IC 5601 is connected to the first wiring 5611, the secondwiring 5612, the third wiring 5613, and the wirings 5621_1 to 5621_M.Each of the switch groups 5602_1 to 5602_M is connected to the firstwiring 5611, the second wiring 5612, and the third wiring 5613. Inaddition, the switch groups 5602_1 to 5602_M are connected to thewirings 5621_1 to 5621_M, respectively. Each of the wirings 5621_1 to5621_M is connected to three signal lines through the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c. For example, the wiring 5621_J of the J-thcolumn (one of the wirings 5621_1 to 5621_M) is connected to a signalline Sj−1, a signal line Sj, and a signal line Sj+1 through the firstthin film transistor 5603 a, the second thin film transistor 5603 b, andthe third thin film transistor 5603 c of the switch group 5602_J.

Note that a signal is input to each of the first wiring 5611, the secondwiring 5612, and the third wiring 5613.

Note that the driver IC 5601 is preferably formed on a single-crystalsubstrate. The switch groups 5602_1 to 5602_M are preferably formed overthe same substrate as the pixel portion. Therefore, the driver IC 5601is preferably connected to the switch groups 5602_1 to 5602_M through anFPC or the like.

Next, operation of the signal line driver circuit of FIG. 12 isdescribed with reference to a timing chart of FIG. 13. FIG. 13illustrates the timing chart where a scan line Gi in the i-th row isselected. A selection period of the scan line Gi in the i-th row isdivided into a first sub-selection period T1, a second sub-selectionperiod T2, and a third sub-selection period T3. In addition, the signalline driver circuit of FIG. 12 operates similarly to FIG. 13 when a scanline in another row is selected.

Note that the timing chart of FIG. 13 shows the case where the wiring5621_J in the J-th column is connected to the signal line Sj−1, thesignal line Sj, and the signal line Sj+1 through the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c.

The timing chart of FIG. 13 shows timing when the scan line Gi in thei-th row is selected, timing 5703 a when the first thin film transistor5603 a is turned on/off, timing 5703 b when the second thin filmtransistor 5603 b is turned on/off, timing 5703 c when the third thinfilm transistor 5603 c is turned on/off, and a signal 5721_J input tothe wiring 5621_J in the J-th column.

In the first sub-selection period T1, the second sub-selection periodT2, and the third sub-selection period T3, different video signals areinput to the wirings 5621_1 to 5621_M. For example, a video signal inputto the wiring 5621_J in the first sub-selection period T1 is input tothe signal line Sj-1, a video signal input to the wiring 5621_J in thesecond sub-selection period T2 is input to the signal line Sj, and avideo signal input to the wiring 5621_J in the third sub-selectionperiod T3 is input to the signal line Sj+1. The video signals input tothe wiring 5621_J in the first sub-selection period T1, the secondsub-selection period T2, and the third sub-selection period T3 aredenoted by Data_j−1, Data_j, and Data_j+1, respectively.

As shown in FIG. 13, in the first sub-selection period T1, the firstthin film transistor 5603 a is on, and the second thin film transistor5603 b and the third thin film transistor 5603 c are off. At this time,Data_j−1 input to the wiring 5621_J is input to the signal line Sj−1through the first thin film transistor 5603 a. In the secondsub-selection period T2, the second thin film transistor 5603 b is on,and the first thin film transistor 5603 a and the third thin filmtransistor 5603 c are off. At this time, Data_j input to the wiring5621_J is input to the signal line Sj through the second thin filmtransistor 5603 b. In the third sub-selection period T3, the third thinfilm transistor 5603 c is on, and the first thin film transistor 5603 aand the second thin film transistor 5603 b are off. At this time,Data_j+1 input to the wiring 5621_J is input to the signal line Sj+1through the third thin film transistor 5603 c.

As described above, in the signal line driver circuit of FIG. 12, onegate selection period is divided into three; thus, video signals can beinput to three signal lines from one wiring 5621 in one gate selectionperiod. Therefore, in the signal line driver circuit of FIG. 12, thenumber of connections between the substrate provided with the driver IC5601 and the substrate provided with the pixel portion can be reduced toapproximately one third the number of signal lines. When the number ofconnections is reduced to approximately one third the number of signallines, the reliability, yield, and the like of the signal line drivercircuit of FIG. 12 can be improved.

Note that there is no particular limitation on the arrangement, number,driving method, and the like of the thin film transistors, as long asone gate selection period is divided into a plurality of sub-selectionperiods and video signals are input to a plurality of signal lines fromone wiring in the respective sub-selection periods as shown in FIG. 12.

For example, when video signals are input to three or more signal linesfrom one wiring in the respective sub-selection periods, it is onlynecessary to add a thin film transistor and a wiring for controlling thethin film transistor. Note that when one gate selection period isdivided into four or more sub-selection periods, one sub-selectionperiod becomes short. Therefore, one gate selection period is preferablydivided into two or three sub-selection periods.

As another example, as shown in a timing chart of FIG. 14, one selectionperiod may be divided into a precharge period Tp, the firstsub-selection period T1, the second sub-selection period T2, and thethird sub-selection period T3. The timing chart of FIG. 14 shows timingwhen the scan line Gi in the i-th row is selected, timing 5803 a whenthe first thin film transistor 5603 a is turned on/off, timing 5803 bwhen the second thin film transistor 5603 b is turned on/off, timing5803 c when the third thin film transistor 5603 c is turned on/off, anda signal 5821_J input to the wiring 5621_J in the J-th column. As shownin FIG. 14, the first thin film transistor 5603 a, the second thin filmtransistor 5603 b, and the third thin film transistor 5603 c are on inthe precharge period Tp. At this time, a precharge voltage Vp input tothe wiring 5621_J is input to the signal line Sj−1, the signal line Sj,and the signal line Sj+1 through the first thin film transistor 5603 a,the second thin film transistor 5603 b, and the third thin filmtransistor 5603 c, respectively. In the first sub-selection period T1,the first thin film transistor 5603 a is on, and the second thin filmtransistor 5603 b and the third thin film transistor 5603 c are off. Atthis time, Data_j−1 input to the wiring 5621_J is input to the signalline Sj−1 through the first thin film transistor 5603 a. In the secondsub-selection period T2, the second thin film transistor 5603 b is on,and the first thin film transistor 5603 a and the third thin filmtransistor 5603 c are off. At this time, Data_j input to the wiring5621_J is input to the signal line Sj through the second thin filmtransistor 5603 b. In the third sub-selection period T3, the third thinfilm transistor 5603 c is on, and the first thin film transistor 5603 aand the second thin film transistor 5603 b are off. At this time,Data_j+1 input to the wiring 5621_J is input to the signal line Sj+1through the third thin film transistor 5603 c.

As described above, in the signal line driver circuit of FIG. 12, towhich the timing chart of FIG. 14 is applied, the signal line can beprecharged by providing the precharge period before the sub-selectionperiods. Thus, a video signal can be written to a pixel with high speed.Note that portions in FIG. 14 which are similar to those in FIG. 13 aredenoted by the same reference numerals, and detailed description of thesame portions or portions having similar functions is omitted.

Now, a constitution of the scan line driver circuit is described. Thescan line driver circuit includes a shift register and a buffer. Also,the scan line driver circuit may include a level shifter in some cases.In the scan line driver circuit, when a clock signal (CLK) and a startpulse signal (SP) are input to the shift register, a selection signal isproduced. The generated selection signal is buffered and amplified bythe buffer, and the resulting signal is supplied to a corresponding scanline. Gate electrodes of transistors in pixels in one line are connectedto the scan line. Further, since the transistors in the pixels in oneline have to be turned on at the same time, a buffer which can feed alarge amount of current is used.

An example of a shift register used as part of the scan line drivercircuit is described with reference to FIG. 15 and FIG. 16.

FIG. 15 illustrates a circuit configuration of the shift register. Theshift register shown in FIG. 15 includes a plurality of flip-flops,flip-flops 5701_1 to 5701 _(—) n. Further, the shift register operatesby input of a first clock signal, a second clock signal, a start pulsesignal, and a reset signal.

Connection relationships of the shift register of FIG. 15 are described.In the flip-flop 5701 _(—) i (one of the flip-flops 5701_1 to 5701 _(—)n) of the i-th stage in the shift register of FIG. 15, a first wiring5501 shown in FIG. 16 is connected to a seventh wiring 5717 _(—) i−1; asecond wiring 5502 shown in FIG. 16 is connected to a seventh wiring5717 _(—) i+1; a third wiring 5503 shown in FIG. 16 is connected to aseventh wiring 5717 _(—) i; and a sixth wiring 5506 shown in FIG. 16 isconnected to a fifth wiring 5715.

Further, a fourth wiring 5504 shown in FIG. 16 is connected to a secondwiring 5712 in flip-flops of odd-numbered stages, and is connected to athird wiring 5713 in flip-flops of even-numbered stages. A fifth wiring5505 shown in FIG. 16 is connected to a fourth wiring 5714.

Note that the first wiring 5501 shown in FIG. 16 of the flip-flop 5701_1of a first stage is connected to a first wiring 5711, and the secondwiring 5502 shown in FIG. 16 of the flip-flop 5701 _(—) n of an n-thstage is connected to a sixth wiring 5716.

The first wiring 5711, the second wiring 5712, the third wiring 5713,and the sixth wiring 5716 may be referred to as a first signal line, asecond signal line, a third signal line, and a fourth signal line,respectively. The fourth wiring 5714 and the fifth wiring 5715 may bereferred to as a first power supply line and a second power supply line,respectively.

FIG. 16 illustrates the detail of the flip-flop shown in FIG. 15. Aflip-flop shown in FIG. 16 includes a first thin film transistor 5571, asecond thin film transistor 5572, a third thin film transistor 5573, afourth thin film transistor 5574, a fifth thin film transistor 5575, asixth thin film transistor 5576, a seventh thin film transistor 5577,and an eighth thin film transistor 5578. Note that the first thin filmtransistor 5571, the second thin film transistor 5572, the third thinfilm transistor 5573, the fourth thin film transistor 5574, the fifththin film transistor 5575, the sixth thin film transistor 5576, theseventh thin film transistor 5577, and the eighth thin film transistor5578 are n-channel transistors, and are turned on when the gate-sourcevoltage (V_(gs)) exceeds the threshold voltage (V_(th)).

Now, a connection structure of the flip-flop shown in FIG. 16 isdescribed below.

A first electrode (one of a source electrode or a drain electrode) ofthe first thin film transistor 5571 is connected to the fourth wiring5504, and a second electrode (the other of the source electrode or thedrain electrode) of the first thin film transistor 5571 is connected tothe third wiring 5503.

A first electrode of the second thin film transistor 5572 is connectedto the sixth wiring 5506. A second electrode of the second thin filmtransistor 5572 is connected to the third wiring 5503.

A first electrode of the third thin film transistor 5573 is connected tothe fifth wiring 5505. A second electrode of the third thin filmtransistor 5573 is connected to a gate electrode of the second thin filmtransistor 5572. A gate electrode of the third thin film transistor 5573is connected to the fifth wiring 5505.

A first electrode of the fourth thin film transistor 5574 is connectedto the sixth wiring 5506. A second electrode of the fourth thin filmtransistor 5574 is connected to the gate electrode of the second thinfilm transistor 5572. A gate electrode of the fourth thin filmtransistor 5574 is connected to a gate electrode of the first thin filmtransistor 5571.

A first electrode of the fifth thin film transistor 5575 is connected tothe fifth wiring 5505. A second electrode of the fifth thin filmtransistor 5575 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the fifth thin film transistor5575 is connected to the first wiring 5501.

A first electrode of the sixth thin film transistor 5576 is connected tothe sixth wiring 5506. A second electrode of the sixth thin filmtransistor 5576 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the sixth thin film transistor5576 is connected to the gate electrode of the second thin filmtransistor 5572.

A first electrode of the seventh thin film transistor 5577 is connectedto the sixth wiring 5506. A second electrode of the seventh thin filmtransistor 5577 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the seventh thin filmtransistor 5577 is connected to the second wiring 5502. A firstelectrode of the eighth thin film transistor 5578 is connected to thesixth wiring 5506. A second electrode of the eighth thin film transistor5578 is connected to the gate electrode of the second thin filmtransistor 5572. A gate electrode of the eighth thin film transistor5578 is connected to the first wiring 5501.

Note that the point at which the gate electrode of the first thin filmtransistor 5571, the gate electrode of the fourth thin film transistor5574, the second electrode of the fifth thin film transistor 5575, thesecond electrode of the sixth thin film transistor 5576, and the secondelectrode of the seventh thin film transistor 5577 are connected isreferred to as a node 5543. The point at which the gate electrode of thesecond thin film transistor 5572, the second electrode of the third thinfilm transistor 5573, the second electrode of the fourth thin filmtransistor 5574, the gate electrode of the sixth thin film transistor5576, and the second electrode of the eighth thin film transistor 5578are connected is referred to as a node 5544.

The first wiring 5501, the second wiring 5502, the third wiring 5503,and the fourth wiring 5504 may be referred to as a first signal line, asecond signal line, a third signal line, and a fourth signal line,respectively. The fifth wiring 5505 and the sixth wiring 5506 may bereferred to as a first power supply line and a second power supply line,respectively.

Alternatively, the signal line driver circuit and the scan line drivercircuit can be manufactured using only n-channel TFTs which can bemanufactured by a method similar to the method for forming thenon-linear element and together with the non-linear element described inEmbodiment 2. Since the n-channel TFTs which can be formed by a methodsimilar to the method for forming the non-linear element and togetherwith the non-linear element described in Embodiment 2 have highmobility, the driving frequency of the driver circuits can be increased.For example, the scan line driver circuit including the n-channel TFTswhich can be formed by a method similar to the method for forming thenon-linear element and together with the non-linear element described inEmbodiment 2 can operate at high speed; therefore, it is possible toincrease the frame frequency or to achieve insertion of a black screen,for example.

In addition, when the channel width of the transistor in the scan linedriver circuit is increased or a plurality of scan line driver circuitsare provided, for example, higher frame frequency can be realized. Whena plurality of scan line driver circuits is provided, a scan line drivercircuit for driving even-numbered scan lines is provided on one side anda scan line driver circuit for driving odd-numbered scan lines isprovided on the opposite side; thus, increase in frame frequency can berealized. Furthermore, the use of the plurality of scan line drivercircuits for output of signals to the same scan line is advantageous inincreasing the size of a display device.

In the case of manufacturing an active matrix light-emitting displaydevice, which is an example of a semiconductor device to which anembodiment of the present invention is applied, a plurality of scan linedriver circuits are preferably arranged because a plurality of thin filmtransistors are arranged in at least one pixel. An example of a blockdiagram of an active matrix light-emitting display device is illustratedin FIG. 11B.

The light-emitting display device illustrated in FIG. 11B includes, overa substrate 5400, a pixel portion 5401 including a plurality of pixelseach provided with a display element, a first scan line driver circuit5402 and a second scan line driver circuit 5404 that select a pixel, anda signal line driver circuit 5403 that controls a video signal input tothe selected pixel.

In the case of inputting a digital video signal to the pixel of thelight-emitting display device of FIG. 11B, the pixel is put in alight-emitting state or non-light-emitting state by switching on/off ofthe transistor. Thus, grayscale can be displayed using an area ratiograyscale method or a time ratio grayscale method. An area ratiograyscale method refers to a driving method by which one pixel isdivided into a plurality of subpixels and the respective subpixels aredriven separately based on video signals so that grayscale is displayed.Further, a time ratio grayscale method refers to a driving method bywhich a period during which a pixel is in a light-emitting state iscontrolled so that grayscale is displayed.

Since the response time of light-emitting elements is shorter than thatof liquid crystal elements or the like, the light-emitting elements aresuitable for a time ratio grayscale method. Specifically, in the case ofdisplaying by a time grayscale method, one frame period is divided intoa plurality of subframe periods. Then, in accordance with video signals,the light-emitting element in the pixel is put in a light-emitting stateor a non-light-emitting state in each subframe period. By dividing aframe into a plurality of subframes, the total length of time in whichpixels actually emit light in one frame period can be controlled withvideo signals to display grayscales.

Note that in the light-emitting display device of FIG. 11B, in the casewhere one pixel includes two switching TFTs, a signal which is input toa first scan line which is a gate wiring of one of the switching TFTs isgenerated in the first scan line driver circuit 5402 and a signal whichis input to a second scan line which is a gate wiring of the otherswitching TFT is generated in the second scan line driver circuit 5404.However, both of the signals which are input to the first scan line andthe second scan line may be generated in one scan line driver circuit.In addition, for example, there is a possibility that a plurality ofscan lines used for controlling the operation of the switching elementsbe provided in each pixel depending on the number of switching TFTsincluded in one pixel. In this case, the signals which are input to thescan lines may all be generated in one scan line driver circuit or maybe generated in a plurality of scan line driver circuits.

Also in the light-emitting display device, part of the driver circuitwhich can be formed using the n-channel TFTs can be provided over onesubstrate together with the thin film transistors of the pixel portion.Moreover, the signal line driver circuit and the scan line drivercircuit can be manufactured using only the n-channel TFTs which can beformed by a method similar to the method for forming the non-linearelement described in Embodiment 2

The above driver circuit may be used for not only a liquid crystaldisplay device or a light-emitting display device but also an electronicpaper in which electronic ink is driven by utilizing an elementelectrically connected to a switching element. The electronic paper isalso called an electrophoretic display device (electrophoretic display)and has advantages in that it has the same level of readability asregular paper, it has less power consumption than other display devices,and it can be made thin and lightweight.

There are a variety of modes of electrophoretic displays. Theelectrophoretic display is a device in which a plurality ofmicrocapsules each including first particles having positive charge andsecond particles having negative charge are dispersed in a solvent or asolute, and an electrical field is applied to the microcapsules so thatthe particles in the microcapsules move in opposite directions from eachother, and only a color of the particles gathered on one side isdisplayed. Note that the first particles or the second particles includea colorant, and does not move when there is not electric field. Also, acolor of the first particles is different from a color of the secondparticles (the particles may also be colorless).

Thus, the electrophoretic display utilizes a so-called dielectrophoreticeffect, in which a substance with high dielectric constant moves to aregion with high electric field. The electrophoretic display does notrequire a polarizing plate and a counter substrate, which are necessaryfor a liquid crystal display device, so that the thickness and weightthereof are about half.

In electronic ink the microcapsules are dispersed in a solvent, and thiselectronic ink can be printed on a surface of glass, plastic, fabric,paper, or the like. Color display is also possible with the use of acolor filter or particles including a coloring matter.

In addition, an active matrix display device can be completed byproviding as appropriate, a plurality of the microcapsules over anactive matrix substrate so as to be interposed between two electrodes,and can perform display by application of electric field to themicrocapsules. For example, the active matrix substrate obtained usingthe thin film transistors which can be formed by a method similar to themethod for forming the non-linear element described in Embodiment 2 canbe used.

Note that the first particles and the second particles in themicrocapsules may be formed from one of a conductive material, aninsulating material, a semiconductor material, a magnetic material, aliquid crystal material, a ferroelectric material, an electroluminescentmaterial, an electrochromic material, and a magnetophoretic material ora composite material thereof.

The protective circuit mounted in thus manufactured display device hasless contact holes for connection, and therefore, occupies a smallerarea. Additionally, in the protective circuit, a second oxidesemiconductor layer which has higher electrical conductivity than afirst oxide semiconductor layer is provided between the first oxidesemiconductor layer and a wiring layer, which realizes enhancement offunction and stable operation of the protective circuit. Therefore, thedisplay device of this embodiment which includes such a protectivecircuit is highly reliable.

This embodiment can be implemented in combination with a structuredescribed in another embodiment as appropriate.

Embodiment 5

A thin film transistor can be manufactured together with a non-linearelement according to an embodiment of the present invention, and thethin film transistor can be used for a pixel portion and further for adriver circuit, so that a semiconductor device having a display function(also called a display device) can be manufactured. Moreover, a thinfilm transistor and a non-linear element according to an embodiment ofthe present invention can be used for part of a driver circuit or anentire driver circuit formed over one substrate together with a pixelportion, so that a system-on-panel can be formed.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. A light-emitting elementincludes, in its category, an element whose luminance is controlled bycurrent or voltage, and specifically includes an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Further, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

In addition, the display device includes a panel in which a displayelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel. An embodiment of the presentinvention further relates to one mode of an element substrate before thedisplay element is completed in a process for manufacturing the displaydevice, and the element substrate is provided with a plurality of pixelseach having a means for supplying current to the display element.Specifically, the element substrate may be in a state after only a pixelelectrode of the display element is formed, a state after a conductivefilm to be a pixel electrode is formed but before the conductive film isetched to be the pixel electrode, or any other states.

A display device in this specification refers to an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device also includes any of the followingmodules in its category: a module to which a connector such as aflexible printed circuit (FPC), a tape automated bonding (TAB) tape, ora tape carrier package (TCP) is attached; a module having a TAB tape ora TCP at the end of which a printed wiring board is provided; and amodule in which an integrated circuit (IC) is directly mounted on adisplay element by a chip-on-glass (COG) method.

The appearance and a cross section of a liquid crystal display panelwhich is one mode of a display device according to an embodiment of thepresent invention will be described in this embodiment with reference toFIGS. 17A1, 17A2, and 17B. FIGS. 17A1 and 17A2 are top views of panelsin which thin film transistors 4010 and 4011 and a liquid crystalelement 4013 which are formed over a first substrate 4001 are sealedwith a sealant 4005 between the first substrate 4001 and a secondsubstrate 4006. The thin film transistors 4010 and 4011 can be formedsimilarly to the non-linear element and each include a semiconductorlayer including a semiconductor oxide containing In, Ga, and Zn and haveexcellent electrical characteristics. FIG. 17B is a cross-sectional viewtaken along M-N of FIGS. 17A1 and 17A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Thus, the pixelportion 4002 and the scan line driver circuit 4004 as well as a liquidcrystal layer 4008 are sealed with the sealant 4005 between the firstsubstrate 4001 and the second substrate 4006. A signal line drivercircuit 4003 that is formed using a single crystal semiconductor film ora polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on a connection method ofthe driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used. FIG. 17A1illustrates an example in which the signal line driver circuit 4003 ismounted by a COG method and FIG. 17A2 illustrates an example in whichthe signal line driver circuit 4003 is mounted by a TAB method.

Each of the pixel portion 4002 and the scan line driver circuit 4004which are provided over the first substrate 4001 includes a plurality ofthin film transistors. FIG. 17B illustrates the thin film transistor4010 included in the pixel portion 4002 and the thin film transistor4011 included in the scan line driver circuit 4004. Insulating layers4020 and 4021 are provided over the thin film transistors 4010 and 4011.

Each of the thin film transistors 4010 and 4011 has excellent electricalcharacteristics and includes a semiconductor layer including asemiconductor oxide containing In, Ga, and Zn. Additionally, the thinfilm transistors 4010 and 4011 can be formed by a method similar to themethod for forming the non-linear element and together with thenon-linear element described in Embodiment 2. In this embodiment, thethin film transistors 4010 and 4011 are n-channel thin film transistors.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. A portion where the pixel electrode layer 4030,the counter electrode layer 4031, and the liquid crystal layer 4008overlap with each other corresponds to the liquid crystal element 4013.Note that the pixel electrode layer 4030 and the counter electrode layer4031 are provided with an insulating layer 4032 and an insulating layer4033 serving as alignment films, respectively, and hold the liquidcrystal layer 4008 with the insulating layers 4032 and 4033 interposedtherebetween.

Note that the first substrate 4001 and the second substrate 4006 can beformed from glass, metal (typically, stainless steel), ceramic, orplastic. As plastic, a fiberglass-reinforced plastics (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. Alternatively, a sheet with a structure in which analuminum foil is sandwiched between PVF films or polyester films can beused.

Reference numeral 4035 denotes a columnar spacer obtained by selectivelyetching an insulating film and is provided to control the distance (acell gap) between the pixel electrode layer 4030 and the counterelectrode layer 4031. Alternatively, a spherical spacer may be used.

Alternatively, a blue phase liquid crystal for which an alignment filmis unnecessary may be used. A blue phase is a type of liquid crystalphase, which appears just before a cholesteric liquid crystal changesinto an isotropic phase when the temperature of the cholesteric liquidcrystal is increased. A blue phase appears only within a narrowtemperature range; therefore, the liquid crystal layer 4008 is formedusing a liquid crystal composition containing a chiral agent at 5 wt. %or more in order to expand the temperature range. The liquid crystalcomposition including a blue phase liquid crystal and a chiral agent hasa short response time of 10 μs to 100 μs, and is optically isotropic;therefore, alignment treatment is not necessary and viewing angledependence is small.

Note that this embodiment describes an example of a transmissive liquidcrystal display device; however, an embodiment of the present inventioncan be applied to a reflective liquid crystal display device or atransflective liquid crystal display device.

Although a liquid crystal display device of this embodiment has apolarizing plate provided outer than the substrate (the viewer side) anda coloring layer and an electrode layer of a display element providedinner than the substrate, which are arranged in that order, thepolarizing plate may be inner than the substrate. The stacked structureof the polarizing plate and the coloring layer is not limited to thatshown in this embodiment and may be set as appropriate in accordancewith the materials of the polarizing plate and the coloring layer andthe condition of the manufacturing process. Further, a light-blockingfilm serving as a black matrix may be provided.

In this embodiment, in order to reduce the unevenness of the surface ofthe thin film transistors and to improve the reliability of the thinfilm transistors, the non-linear element described in Embodiment 2 andthe thin film transistors which can be formed by a method similar to themethod for forming the non-linear element and can be formed togetherwith the non-linear element are covered with protective films orinsulating layers (the insulating layers 4020 and 4021) serving asplanarizing insulating films. Note that the protective film is providedto prevent entry of impurities floating in air, such as an organicsubstance, a metal substance, or moisture, and is preferably a densefilm. The protective film may be formed by a sputtering method using asingle layer or a stack of layers of a silicon oxide film, a siliconnitride film, a silicon oxynitride film, a silicon nitride oxide film,an aluminum oxide film, an aluminum nitride film, an aluminum oxynitridefilm, or an aluminum nitride oxide film. Although the protective film isformed by a sputtering method in this embodiment, the method is notlimited to a particular method and may be selected from a variety ofmethods.

Here, the insulating layer 4020 is formed to have a stacked structure asthe protective film. Here, a silicon oxide film is formed by asputtering method as a first layer of the insulating layer 4020. The useof a silicon oxide film for the protective film provides an advantageouseffect of preventing hillock of an aluminum film used for a sourceelectrode layer and a drain electrode layer.

Moreover, an insulating layer is formed as a second layer of theprotective film. Here, a silicon nitride film is formed by a sputteringmethod as a second layer of the insulating layer 4020. When a siliconnitride film is used for the protective film, it is possible to preventmovable ions such as sodium from entering a semiconductor region tochange the electrical characteristics of the TFT.

Further, after the protective film is formed, the oxide semiconductorlayer may be annealed (at 300° C. to 400° C.).

Further, the insulating layer 4021 is formed as the planarizinginsulating film. The insulating layer 4021 can be formed from an organicmaterial having heat resistance, such as polyimide, acrylic,benzocyclobutene, polyamide, or epoxy. As an alternative to such organicmaterials, it is possible to use a low-dielectric constant material (alow-k material), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like. A siloxane-based resinmay include as a substituent at least one of fluorine, an alkyl group,and an aryl group, as well as hydrogen. Note that the insulating layer4021 may be formed by stacking a plurality of insulating films formedfrom these materials.

Note that a siloxane-based resin is a resin formed from a siloxane-basedmaterial as a starting material and having the bond of Si—O—Si. Thesiloxane-based resin may include as a substituent at least one offluorine, an alkyl group, and aromatic hydrocarbon, as well as hydrogen.

The method for the formation of the insulating layer 4021 is not limitedto a particular method and the following method can be used depending onthe material of the insulating layer 4021: a sputtering method, an SOGmethod, spin coating, dip coating, spray coating, a droplet dischargemethod (e.g., an inkjet method, screen printing, or offset printing), adoctor knife, a roll coater, a curtain coater, a knife coater, or thelike. In the case of forming the insulating layer 4021 with the use of amaterial solution, annealing (at 300° C. to 400° C.) may be performed onan oxide semiconductor layer at the same time as a baking step. When thebaking step of the insulating layer 4021 and the annealing of the oxidesemiconductor layer are combined, a semiconductor device can bemanufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed from a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

A conductive composition including a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed of the conductive composition has preferably a sheet resistanceof 10000 Ω/square or less and a transmittance of 70% or more at awavelength of 550 nm. Further, the resistivity of the conductive highmolecule included in the conductive composition is preferably 0.1Ω·cm orless.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. As examples thereof, polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, a copolymer of two or more of them can begiven.

Further, a variety of signals and potentials are supplied from an FPC4018 to the signal line driver circuit 4003 which is formed separately,the scan line driver circuit 4004, and the pixel portion 4002.

In this embodiment, a connecting terminal electrode 4015 is formed usingthe same conductive film as the pixel electrode layer 4030 included inthe liquid crystal element 4013, and a terminal electrode 4016 is formedusing the same conductive film as the source and drain electrode layersof the thin film transistors 4010 and 4011.

The connecting terminal electrode 4015 is electrically connected to aterminal of the FPC 4018 through an anisotropic conductive film 4019.

Although FIGS. 17A1, 17A2, and 17B show an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, this embodiment is not limited to this structure. Thescan line driver circuit may be separately formed and then mounted, oronly part of the signal line driver circuit or part of the scan linedriver circuit may be separately formed and then mounted.

FIG. 18 illustrates an example in which a liquid crystal display moduleis formed as a semiconductor device using a TFT substrate 2600manufactured according to an embodiment of the present invention.

FIG. 18 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including a TFTand the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605 are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the case of the RGB system, coloring layers corresponding tocolors of red, green, and blue are provided for pixels. Polarizingplates 2606 and 2607 and a diffuser plate 2613 are provided outside theTFT substrate 2600 and the counter substrate 2601. A light sourceincludes a cold cathode tube 2610 and a reflective plate 2611, and acircuit board 2612 is connected to a wiring circuit portion 2608 of theTFT substrate 2600 through a flexible wiring board 2609 and includes anexternal circuit such as a control circuit and a power source circuit.The polarizing plate and the liquid crystal layer may be stacked with aretardation plate interposed therebetween.

For the liquid crystal display module, a TN (twisted nematic) mode, anIPS (in-plane-switching) mode, an FFS (fringe field switching) mode, anMVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optically compensated birefringence) mode, an FLC (fenoelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be employed.

The protective circuit mounted in thus manufactured liquid crystal panelhas less contact holes for connection, and therefore, occupies a smallerarea. Additionally, in the protective circuit, a second oxidesemiconductor layer which has higher electrical conductivity than afirst oxide semiconductor layer is provided between the first oxidesemiconductor layer and a wiring layer, which realizes enhancement offunction and stable operation of the protective circuit. Therefore, theliquid crystal panel of this embodiment which includes such a protectivecircuit is highly reliable.

This embodiment can be implemented in combination with a structuredescribed in another embodiment as appropriate.

Embodiment 6

According to an embodiment of the present invention, a thin filmtransistor can be manufactured together with a non-linear element andthe thin film transistor can be used for a pixel portion and further fora driver circuit, so that a semiconductor device having a displayfunction (also called a display device) can be manufactured.

This embodiment describes an example of a light-emitting display deviceas a display device according to an embodiment of the present invention.As an example of a display element of the display device, here, alight-emitting element utilizing electroluminescence is described.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of a voltage to alight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and thus current flows. Then, those carriers (i.e., electronsand holes) are recombined, and thus, the light-emitting organic compoundis excited. When the light-emitting organic compound returns to a groundstate from the excited state, light is emitted. Owing to such amechanism, this light emitting element is referred to as acurrent-excitation light emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion type inorganic EL element and a thin-filmtype inorganic EL element. A dispersion type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film type inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an organic ELelement is used as a light-emitting element in this example.

FIG. 19 illustrates an example of a pixel structure to which digitaltime grayscale driving can be applied, as an example of a semiconductordevice to which an embodiment of the present invention is applied.

A structure and operation of a pixel to which digital time grayscaledriving can be applied are described. In this example, one pixelincludes two n-channel transistors in each of which a channel formationregion includes an oxide semiconductor layer and which can be formed bya method similar to the method for forming the non-linear elementdescribed in Embodiment 2.

A pixel 6400 includes a switching transistor 6401, a driver transistor6402, a light-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scan line 6406, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) of the switching transistor 6401 is connected to a gate ofthe driver transistor 6402. The gate of the driver transistor 6402 isconnected to a power supply line 6407 through the capacitor 6403, afirst electrode of the driver transistor 6402 is connected to the powersupply line 6407, and a second electrode of the driver transistor 6402is connected to a first electrode (pixel electrode) of thelight-emitting element 6404. A second electrode of the light-emittingelement 6404 corresponds to a common electrode 6408.

The second electrode (common electrode 6408) of the light-emittingelement 6404 is set to a low power supply potential. The low powersupply potential is a potential smaller than a high power supplypotential when the high power supply potential set to the power supplyline 6407 is a reference. As the low power supply potential, GND, 0 V,or the like may be employed, for example. A potential difference betweenthe high power supply potential and the low power supply potential isapplied to the light-emitting element 6404 and current is supplied tothe light-emitting element 6404, so that the light-emitting element 6404emits light. In order to make the light-emitting element 6404 emitlight, potentials are set so that the potential difference between thehigh power supply potential and the low power supply potential isgreater than or equal to the forward threshold voltage of thelight-emitting element 6404.

Gate capacitance of the driver transistor 6402 may be used as asubstitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitance of the driver transistor 6402 may beformed between the channel region and the gate electrode.

In the case of a voltage-input voltage driving method, a video signal isinput to the gate of the driver transistor 6402 so that the drivertransistor 6402 is either substantially turned on or substantiallyturned off. That is, the driver transistor 6402 operates in a linearregion. Since the driver transistor 6402 operates in a linear region, avoltage higher than the voltage of the power supply line 6407 is appliedto the gate of the driver transistor 6402. Note that a voltage higherthan or equal to the sum of the voltage of the power supply line and theVth of the driver transistor 6402 is applied to the signal line 6405.

In the case of performing analog grayscale driving instead of digitaltime grayscale driving, the same pixel structure as that in FIG. 19 canbe used by changing signal input.

In the case of performing analog grayscale driving, a voltage higherthan or equal to the sum of the forward voltage of the light-emittingelement 6404 and the Vth of the driver transistor 6402 is applied to thegate of the driver transistor 6402. The forward voltage of thelight-emitting element 6404 indicates a voltage at which a desiredluminance is obtained, and includes at least a forward thresholdvoltage. The video signal by which the driver transistor 6402 operatesin a saturation region is input, so that current can be supplied to thelight-emitting element 6404. In order for the driver transistor 6402 tooperate in a saturation region, the potential of the power supply line6407 is set higher than the gate potential of the driver transistor6402. When an analog video signal is used, it is possible to feedcurrent to the light-emitting element 6404 in accordance with the videosignal and perform analog grayscale driving.

Note that the pixel structure shown in FIG. 19 is not limited thereto.For example, a switch, a resistor, a capacitor, a transistor, a logiccircuit, or the like may be added to the pixel shown in FIG. 19.

Next, structures of a light-emitting element are described withreference to FIGS. 20A to 20C. A cross-sectional structure of a pixel isdescribed here by taking an n-channel driver TFT as an example. TFTs7001, 7011, and 7021 serving as driver TFTs used for a semiconductordevice, which are illustrated in FIGS. 20A, 20B, and 20C, can be formedby a method similar to the method for forming the non-linear element andtogether with the non-linear element described in Embodiment 2. The TFTs7001, 7011, and 7021 have excellent electrical characteristics and eachinclude a semiconductor layer including a semiconductor oxide containingIn, Ga, and Zn.

In addition, in order to extract light emitted from the light-emittingelement, at least one of an anode and a cathode is required to transmitlight. A thin film transistor and a light-emitting element are formedover a substrate. A light-emitting element can have a top-emissionstructure in which light is extracted through the surface opposite tothe substrate; a bottom-emission structure in which light is extractedthrough the surface on the substrate side; or a dual-emission structurein which light is extracted through the surface opposite to thesubstrate and the surface on the substrate side. The pixel structureaccording to an embodiment of the present invention can be applied to alight-emitting element having any of these emission structures.

A light-emitting element with a top-emission structure is described withreference to FIG. 20A.

FIG. 20A is a cross-sectional view of a pixel in a case where the TFT7001 serving as a driver TFT is an n-channel TFT and light generated ina light-emitting element 7002 is emitted to an anode 7005 side. In FIG.20A, a cathode 7003 of the light-emitting element 7002 is electricallyconnected to the TFT 7001 serving as a driver TFT, and a light-emittinglayer 7004 and the anode 7005 are stacked in this order over the cathode7003. The cathode 7003 can be formed using any of conductive materialswhich have a low work function and reflect light. For example, Ca, Al,MgAg, AlLi, or the like is preferably used. The light-emitting layer7004 may be formed using a single layer or by stacking a plurality oflayers. When the light-emitting layer 7004 is formed using a pluralityof layers, the light-emitting layer 7004 is formed by stacking anelectron-injecting layer, an electron-transporting layer, alight-emitting layer, a hole-transporting layer, and a hole-injectinglayer in this order over the cathode 7003. It is not necessary to formall of these layers. The anode 7005 is formed using a light-transmittingconductive film formed from a light-transmitting conductive materialsuch as indium oxide including tungsten oxide, indium zinc oxideincluding tungsten oxide, indium oxide including titanium oxide, indiumtin oxide including titanium oxide, indium tin oxide (hereinafter,referred to as ITO), indium zinc oxide, or indium tin oxide to whichsilicon oxide is added.

The light-emitting element 7002 corresponds to a region where thecathode 7003 and the anode 7005 sandwich the light-emitting layer 7004.In the case of the pixel illustrated in FIG. 20A, light is emitted fromthe light-emitting element 7002 to the anode 7005 side as indicated byan arrow.

Next, a light-emitting element having a bottom-emission structure isdescribed with reference to FIG. 20B. FIG. 20B is a cross-sectional viewof a pixel in the case where a driver TFT 7011 is an n-channel TFT, andlight is emitted from a light-emitting element 7012 to a cathode 7013side. In FIG. 20B, the cathode 7013 of the light-emitting element 7012is formed over a light-transmitting conductive film 7017 which iselectrically connected to the driver TFT 7011, and a light-emittinglayer 7014 and an anode 7015 are stacked in this order over the cathode7013. A light-blocking film 7016 for reflecting or blocking light may beformed so as to cover the anode 7015 when the anode 7015 has alight-transmitting property. For the cathode 7013, any of conductivematerials which have a low work function can be used as in the case ofFIG. 20A. Note that the cathode 7013 is formed to have a thickness withwhich the cathode 7013 can transmit light (preferably, approximatelyfrom 5 nm to 30 nm). For example, an aluminum film with a thickness of20 nm can be used as the cathode 7013. The light-emitting layer 7014 maybe formed of a single layer or by stacking a plurality of layers as inthe case of FIG. 20A. The anode 7015 is not required to transmit light,but can be formed using a light-transmitting conductive material as inthe case of FIG. 20A. For the light-blocking film 7016, metal or thelike that reflects light can be used; however, it is not limited to ametal film. For example, a resin or the like to which black pigment isadded can be used.

The light-emitting element 7012 corresponds to a region where thecathode 7013 and the anode 7015 sandwich the light-emitting layer 7014.In the case of the pixel illustrated in FIG. 20B, light is emitted fromthe light-emitting element 7012 to the cathode 7013 side as indicated byan arrow.

Next, a light-emitting element having a dual-emission structure isdescribed with reference to FIG. 20C. In FIG. 20C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the driver TFT7021, and a light-emitting layer 7024 and an anode 7025 are stacked inthis order over the cathode 7023. As in the case of FIG. 20A, thecathode 7023 can be formed of any of conductive materials which have alow work function. Note that the cathode 7023 is formed to have athickness with which the cathode 7023 can transmit light. For example,an Al film having a thickness of 20 nm can be used as the cathode 7023.The light-emitting layer 7024 may be formed using a single layer or bystacking a plurality of layers as in the case of FIG. 20A. In a mannersimilar to FIG. 20A, the anode 7025 can be formed using alight-transmitting conductive material.

The light-emitting element 7022 corresponds to a region where thecathode 7023, the light-emitting layer 7024, and the anode 7025 overlapwith each other. In the pixel illustrated in FIG. 20C, light is emittedfrom the light-emitting element 7022 to both the anode 7025 side and thecathode 7023 side as indicated by arrows.

Although an organic EL element is described here as a light-emittingelement, an inorganic EL element can be alternatively provided as alight-emitting element.

Note that this embodiment describes the example in which a thin filmtransistor (driver TFT) which controls the driving of a light-emittingelement is electrically connected to the light-emitting element, but astructure may be employed in which a current control TFT is connectedbetween the driver TFT and the light-emitting element.

The semiconductor device described in this embodiment is not limited tothe structures illustrated in FIGS. 20A to 20C, and can be modified invarious ways based on the spirit of techniques according to anembodiment of the present invention.

Next, the appearance and cross section of a light-emitting display panel(also referred to as a light-emitting panel) which is one mode of asemiconductor device according to the present invention will bedescribed with reference to FIGS. 21A and 21B. FIG. 21A is a top view ofa panel in which a light-emitting element and a thin film transistor aresealed with a sealant between a first substrate and a second substrate.The thin film transistor includes, a semiconductor layer including asemiconductor oxide containing In, Ga, and Zn, and thus has excellentelectrical characteristics, as the non-linear element. FIG. 21B is across-sectional view along H-I of FIG. 21A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b, which are provided over a first substrate4501. In addition, a second substrate 4506 is formed over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed, together withfiller 4507, with the first substrate 4501, the sealant 4505, and thesecond substrate 4506. In this manner, it is preferable that thelight-emitting display panel be packaged (sealed) with a protective film(such as an attachment film or an ultraviolet curable resin film) or acover material with high air-tightness and little degasification so asnot to be exposed to external air.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of thin film transistors.A thin film transistor 4510 included in the pixel portion 4502 and thethin film transistor 4509 included in the signal line driver circuit4503 a are illustrated as an example in FIG. 21B.

Each of the thin film transistors 4509 and 4510 has excellent electricalcharacteristics and includes a semiconductor layer including asemiconductor oxide containing In, Ga, and Zn. Thin film transistorswhich can be formed by a method similar to the method for forming thenon-linear element and together with the non-linear element described inEmbodiment 2 can be used as the thin film transistors 4509 and 4510. Inthis embodiment, the thin film transistors 4509 and 4510 are n-channelthin film transistors.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a source ordrain electrode layer of the thin film transistor 4510. Note thatalthough the light-emitting element 4511 has a stacked structure of thefirst electrode layer 4517, an electroluminescent layer 4512, and asecond electrode layer 4513, the structure of the light-emitting element4511 is not limited thereto. The structure of the light-emitting element4511 can be changed as appropriate depending on a direction in whichlight is extracted from the light-emitting element 4511, or the like.

A partition wall 4520 is formed using an organic resin film, aninorganic insulating film, or organic polysiloxane. It is particularlypreferable that the partition wall 4520 be formed using a photosensitivematerial to have an opening portion on the first electrode layer 4517 sothat a sidewall of the opening portion is formed as an inclined surfacewith a continuous curvature.

The electroluminescent layer 4512 may be formed using a single layer ora plurality of layers stacked.

In order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide,or the like into the light-emitting element 4511, a protective film maybe formed over the second electrode layer 4513 and the partition wall4520. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

In addition, a variety of signals and potentials are supplied from FPCs4518 a and 4518 b to the signal line driver circuits 4503 a and 4503 b,the scan line driver circuits 4504 a and 4504 b, or the pixel portion4502.

In this embodiment, a connecting terminal electrode 4515 is formed usingthe same conductive film as the first electrode layer 4517 included inthe light-emitting element 4511. A terminal electrode 4516 is formedusing the same conductive film as the source and drain electrode layersincluded in the thin film transistors 4509 and 4510.

The connecting terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivefilm 4519.

The second substrate located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used as well as inert gas such as nitrogen or argon. Forexample, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA)can be used. In this embodiment, nitrogen is used for the filler 4507.

In addition, if needed, an optical film such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),and a color filter may be provided as appropriate on an emission surfaceof the light-emitting element. Further, the polarizing plate or thecircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment can be performed by whichreflected light is diffused by depressions and projections of thesurface and glare can be reduced.

As the signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b, driver circuits formed by using asingle crystal semiconductor film or a polycrystalline semiconductorfilm over a substrate separately prepared may be mounted. In addition,only the signal line driver circuits or only part thereof, or only thescan line driver circuits or only part thereof may be separately formedand then mounted. This embodiment is not limited to the structure shownin FIGS. 21A and 21B.

The protective circuit mounted in thus manufactured a light-emittingdisplay device (display panel) liquid crystal panel has less contactholes for connection, and therefore, occupies a smaller area.Additionally, in the protective circuit, a second oxide semiconductorlayer which has higher electrical conductivity than a first oxidesemiconductor layer is provided between the first oxide semiconductorlayer and a wiring layer, which realizes enhancement of function andstable operation of the protective circuit. Therefore, thelight-emitting display device (display panel) of this embodiment whichincludes such a protective circuit is highly reliable.

This embodiment can be implemented in combination with a structuredescribed in another embodiment as appropriate.

Embodiment 7

A display device according to an embodiment of the present invention canbe applied as an electronic paper. An electronic paper can be used forelectronic appliances of a variety of fields for displaying information.For example, an electronic paper can be used for electronic book reader(an e-book reader), posters, advertisement in vehicles such as trains,displays of various cards such as credit cards, and the like. Examplesof such electronic appliances are illustrated in FIGS. 22A and 22B andFIG. 23.

FIG. 22A illustrates a poster 2631 formed using an electronic paper. Ifthe advertising medium is printed paper, the advertisement is replacedby hands; however, when an electronic paper to which an embodiment ofthe present invention is applied is used, the advertisement display canbe changed in a short time. Moreover, a stable image can be obtainedwithout defects. Further, the poster may send and receive informationwirelessly.

FIG. 22B illustrates an advertisement 2632 in a vehicle such as a train.If the advertising medium is printed paper, the advertisement isreplaced by hands; however, when an electronic paper to which anembodiment of the present invention is applied is used, theadvertisement display can be changed in a short time without muchmanpower. Moreover, a stable image can be obtained without defects.Further, the advertisement in vehicles may send and receive informationwirelessly.

FIG. 23 illustrates an example of an electronic book reader 2700. Forexample, the electronic book reader 2700 includes two housings 2701 and2703. The housings 2701 and 2703 are bonded with a hinge 2711 so thatthe electronic book reader 2700 can be opened and closed along the hinge2711. With such a structure, the electronic book reader 2700 can behandled like a paper book.

A display portion 2705 is incorporated in the housing 2701 and a displayportion 2707 is incorporated in the housing 2703. The display portion2705 and the display portion 2707 may display one image, or may displaydifferent images. In the structure where different images are displayedon the display portion 2705 and the display portion 2707, for example,the right display portion (the display portion 2705 in FIG. 23) candisplay text and the left display portion (the display portion 2707 inFIG. 23) can display images.

FIG. 23 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power supply switch 2721, an operation key 2723, aspeaker 2725, and the like. The page can be turned with the operationkey 2723. Note that a keyboard, a pointing device, and the like may beprovided on the same plane as the display portion of the housing.Further, a rear surface or a side surface of the housing may be providedwith an external connection terminal (an earphone terminal, a USBterminal, a terminal which can be connected with a variety of cablessuch as an AC adopter or a USB cable, and the like), a storage mediuminserting portion, or the like. Moreover, the electronic book reader2700 may have a function of an electronic dictionary.

Further, the electronic book reader 2700 may send and receiveinformation wirelessly. Desired book data or the like can be purchasedand downloaded from an electronic book server wirelessly.

The protective circuit mounted in the display device of this embodimenthas less contact holes for connection, and therefore, occupies a smallerarea. Additionally, in the protective circuit, a second oxidesemiconductor layer which has higher electrical conductivity than afirst oxide semiconductor layer is provided between the first oxidesemiconductor layer and a wiring layer, which realizes enhancement offunction and stable operation of the protective circuit. Therefore, thedisplay device of this embodiment which includes such a protectivecircuit is highly reliable.

This embodiment can be implemented in combination with a structuredescribed in another embodiment as appropriate.

Embodiment 8

A semiconductor device according to an embodiment of the presentinvention can be applied to a variety of electronic appliances(including game machines). As the electronic appliances, for example,there are a television device (also called a television or a televisionreceiver), a monitor for a computer or the like, a camera such as adigital camera or a digital video camera, a digital photo frame, acellular phone (also called a mobile phone or a mobile telephonedevice), a portable game console, a portable information terminal, anaudio playback device, and a large game machine such as a pachinkomachine.

FIG. 24A illustrates an example of a television device 9600. A displayportion 9603 is incorporated in a housing 9601 of the television device9600. The display portion 9603 can display images. Here, the housing9601 is supported on a stand 9605.

The television device 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller 9610. The channel andvolume can be controlled with operation keys 9609 of the remotecontroller 9610 and the images displayed on the display portion 9603 canbe controlled. Moreover, the remote controller 9610 may have a displayportion 9607 on which the information outgoing from the remotecontroller 9610 is displayed.

Note that the television device 9600 is provided with a receiver, amodem, and the like. With the receiver, general television broadcastingcan be received. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (e.g., between a sender and areceiver or between receivers) information communication can beperformed.

FIG. 24B illustrates an example of a digital photo frame 9700. Forexample, a display portion 9703 is incorporated in a housing 9701 of thedigital photo frame 9700. The display portion 9703 can display a varietyof images, for example, displays image data taken with a digital cameraor the like, so that the digital photo frame can function in a mannersimilar to a general picture frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (such as a USB terminal or aterminal which can be connected to a variety of cables including a USBcable), a storage medium inserting portion, and the like. They may beincorporated on the same plane as the display portion; however, they arepreferably provided on a side surface or the rear surface of the displayportion because the design is improved. For example, a memory includingimage data taken with a digital camera is inserted into the storagemedium inserting portion of the digital photo frame and the image datais imported. Then, the imported image data can be displayed on thedisplay portion 9703.

The digital photo frame 9700 may send and receive informationwirelessly. Via wireless communication, desired image data can bewirelessly imported into the digital photo frame 9700 and displayed.

FIG. 25A illustrates a portable game console including a housing 9881and a housing 9891 which are jointed with a connector 9893 so as to beopened and closed. A display portion 9882 and a display portion 9883 areincorporated in the housing 9881 and the housing 9891, respectively. Theportable game console illustrated in FIG. 25A additionally includes aspeaker portion 9884, a storage medium inserting portion 9886, an LEDlamp 9890, an input means (operation keys 9885, a connection terminal9887, a sensor 9888 (having a function of measuring force, displacement,position, speed, acceleration, angular speed, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, vibration, smell, or infraredray), and a microphone 9889), and the like. Needless to say, thestructure of the portable game console is not limited to the above, andmay be any structure which is provided with at least a semiconductordevice according to an embodiment of the present invention. Moreover,another accessory may be provided as appropriate. The portable gameconsole illustrated in FIG. 25A has a function of reading a program ordata stored in a storage medium to display it on the display portion,and a function of sharing information with another portable game consolevia wireless communication. The portable game console of FIG. 25A canhave a variety of functions other than those above.

FIG. 25B illustrates an example of a slot machine 9900, which is a largegame machine. A display portion 9903 is incorporated in a housing 9901of the slot machine 9900. The slot machine 9900 additionally includes anoperation means such as a start lever or a stop switch, a coin slot, aspeaker, and the like. Needless to say, the structure of the slotmachine 9900 is not limited to the above and may be any structure whichis provided with at least a semiconductor device according to anembodiment of the present invention. Moreover, another accessory may beprovided as appropriate.

FIG. 26 illustrates an example of a cellular phone 1000. The cellularphone 1000 includes a housing 1001 in which a display portion 1002 isincorporated, and moreover includes an operation button 1003, anexternal connection port 1004, a speaker 1005, a microphone 1006, andthe like.

Information can be input to the cellular phone 1000 illustrated in FIG.26 by touching the display portion 1002 with a finger or the like.Moreover, calling or text messaging can be performed by touching thedisplay portion 1002 with a finger or the like.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting information such as text. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are mixed.

For example, in the case of calling or text messaging, the displayportion 1002 is set to a text input mode mainly for inputting text, andtext input operation can be performed on a screen. In this case, it ispreferable to display a keyboard or number buttons on almost the entirescreen of the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 1000, display on the screen of the display portion 1002can be automatically switched by judging the direction of the cellularphone 1000 (whether the cellular phone 1000 is placed horizontally orvertically for a landscape mode or a portrait mode).

Further, the screen modes are switched by touching the display portion1002 or operating the operation button 1003 of the housing 1001.Alternatively, the screen modes can be switched depending on kinds ofimages displayed on the display portion 1002. For example, when a signalfor an image displayed on the display portion is data of moving images,the screen mode is switched to the display mode. When the signal is textdata, the screen mode is switched to the input mode.

Further, in the input mode, a signal is detected by an optical sensor inthe display portion 1002 and if input by touching the display portion1002 is not performed for a certain period, the screen mode may becontrolled so as to be switched from the input mode to the display mode.

The display portion 1002 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenby touching the display portion 1002 with the palm or the finger,whereby personal authentication can be performed. Moreover, when abacklight or sensing light source which emits near-infrared light isprovided in the display portion, an image of finger veins, palm veins,or the like can be taken.

The protective circuit mounted in the electronic appliance of thisembodiment has less contact holes for connection, and therefore,occupies a smaller area. Additionally, in the protective circuit, asecond oxide semiconductor layer which has higher electricalconductivity than a first oxide semiconductor layer is provided betweenthe first oxide semiconductor layer and a wiring layer, which realizesenhancement of function and stable operation of the protective circuit.Therefore, the electronic appliance of this embodiment which includessuch a protective circuit is highly reliable.

This embodiment can be implemented in combination with a structuredescribed in another embodiment as appropriate.

This application is based on Japanese Patent Application serial no.2008-259060 filed with Japan Patent Office on Oct. 3, 2008, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a scan line, asignal line, and pixel portion over a substrate; wherein the pixelportion comprises: a gate electrode over the substrate; a gateinsulating film over the gate electrode; an oxide semiconductor layerover the gate electrode; and a source electrode and a drain electrodeover the oxide semiconductor layer; and an insulating film over theoxide semiconductor layer, the source electrode, and the drainelectrode, wherein the insulating film is in contact with a portion ofthe oxide semiconductor layer between the source electrode and the drainelectrode, wherein the gate electrode is electrically connected to thescan line, wherein one of the source electrode and the drain electrodeis electrically connected to the signal line, wherein end portions ofthe source electrode and the drain electrode overlap with the gateelectrode, and wherein the oxide semiconductor layer has a depressedportion, the depressed portion overlaps with the gate electrode.
 2. Thedisplay device according to claim 1, further comprising a second oxidesemiconductor layer between the oxide semiconductor layer and one of thesource electrode and the drain electrode.
 3. The display deviceaccording to claim 1, wherein the oxide semiconductor layer includesindium, gallium, and zinc.
 4. The display device according to claim 2,wherein the second oxide semiconductor layer includes indium, gallium,and zinc.
 5. The display device according to claim 2, wherein the oxidesemiconductor layer has a higher oxygen concentration than the secondoxide semiconductor layer.
 6. The display device according to claim 2,wherein the oxide semiconductor layer has lower electrical conductivitythan the second oxide semiconductor layer.
 7. The display deviceaccording to claim 1, wherein the gate electrode and the scan line areformed of a same material.
 8. The display device according to claim 1,further comprising: a first wiring over the substrate; a semiconductorlayer over the first wiring; and a second wiring and a third wiring overthe semiconductor layer, wherein the second wiring is electricallyconnected to the drain electrode and the first wiring, and wherein thethird wiring is electrically connected to the source electrode and thegate electrode.
 9. A display device comprising: a scan line, a signalline, and pixel portion over a substrate; wherein the pixel portioncomprises: a gate electrode over the substrate; a gate insulating filmover the gate electrode; a first oxide semiconductor layer over the gateelectrode; a second oxide semiconductor layer over the first oxidesemiconductor layer; and a source electrode and a drain electrode overthe second oxide semiconductor layer; and an insulating film over thefirst oxide semiconductor layer, the second oxide semiconductor layer,the source electrode, and the drain electrode, wherein the insulatingfilm is in contact with a portion of the first oxide semiconductor layerbetween the source electrode and the drain electrode, wherein the gateelectrode is electrically connected to the scan line, wherein one of thesource electrode and the drain electrode is electrically connected tothe signal line, wherein end portions of the source electrode and thedrain electrode overlap with the gate electrode.
 10. The display deviceaccording to claim 9, wherein the first oxide semiconductor layer andthe second oxide semiconductor layer includes indium, gallium, and zinc.11. The display device according to claim 9, wherein the first oxidesemiconductor layer has a higher oxygen concentration than the secondoxide semiconductor layer.
 12. The display device according to claim 9,wherein the first oxide semiconductor layer has lower electricalconductivity than the second oxide semiconductor layer.
 13. The displaydevice according to claim 9, wherein the gate electrode and the scanline are formed of a same material.
 14. The display device according toclaim 9, further comprising: a first wiring over the substrate; asemiconductor layer over the first wiring; and a second wiring and athird wiring over the semiconductor layer, wherein the second wiring iselectrically connected to the drain electrode and the first wiring, andwherein the third wiring is electrically connected to the sourceelectrode and the gate electrode.
 15. A display device comprising: ascan line, a signal line, and pixel portion over a substrate; whereinthe pixel portion comprises: a gate electrode over the substrate; a gateinsulating film over the gate electrode; a first oxide semiconductorlayer over the gate electrode; a second oxide semiconductor layer overthe first oxide semiconductor layer; and a source electrode and a drainelectrode over the second oxide semiconductor layer; and an insulatingfilm over the first oxide semiconductor layer, the second oxidesemiconductor layer, the source electrode, and the drain electrode,wherein the insulating film is in contact with a portion of the firstoxide semiconductor layer between the source electrode and the drainelectrode, wherein the gate electrode is electrically connected to thescan line, wherein one of the source electrode and the drain electrodeis electrically connected to the signal line, wherein end portions ofthe source electrode and the drain electrode overlap with the gateelectrode, wherein the first oxide semiconductor layer has a depressedportion, wherein the second oxide semiconductor layer has an opening,and wherein the depressed portion and the opening overlap with the gateelectrode.
 16. The display device according to claim 15, wherein thefirst oxide semiconductor layer and the second oxide semiconductor layerincludes indium, gallium, and zinc.
 17. The display device according toclaim 15, wherein the first oxide semiconductor layer has a higheroxygen concentration than the second oxide semiconductor layer.
 18. Thedisplay device according to claim 15, wherein the first oxidesemiconductor layer has lower electrical conductivity than the secondoxide semiconductor layer.
 19. The display device according to claim 15,wherein the gate electrode and the scan line are formed of a samematerial.
 20. The display device according to claim 15, furthercomprising: a first wiring over the substrate; a semiconductor layerover the first wiring; and a second wiring and a third wiring over thesemiconductor layer, wherein the second wiring is electrically connectedto the drain electrode and the first wiring, and wherein the thirdwiring is electrically connected to the source electrode and the gateelectrode.